For years the internet has taken John Clements belief that edge to edge parrying and blocking is BS because it damages the sword and its better to block with the flat of the sword because the sword gets damaged. Frequently pointed out is how Japanese styles use flat blocks rather than edge to edge blocking.
Now I know in recent years swordsmen, historians, and HEMAists such as Matt Easton are now calling BS on John Clement's claims because not only is there historical evidence of edge to edge defenses to the point even older schools of Japanese swordsmanship have texts showing it, but also because thesword will get damaged anyway and its much quicker and more convenient to block with the edge.
I agree John Clement's claim is BS at least in regards that only the flat should be used in blocking. However I did an experiment recently. I have an Arabian Scimitar short sword and a machete. I had a friend swing the machete at me which I would attempt blocks and parrying with the Scimitar. Now I notice first of all blocking and parrying using the edge DIRECTLY DAMAGED the sword in as early as the first blow and by the time I was finished the sword was so damaged you can see the once smooth blade had big chips in it. Cutting items became much more difficult and attempting to cut wood and meat doesn't slice it evenly anymore. Instead you see a razor zig zagging result with the cut item and what would take a single slice before would take several because the blade gets stuck in the item.
Secondly blocking and parrying was so difficult. Not only did many machete hits directly break through my blade and contacted my body but even successful blocks felt like it took so much of my strength especially single hand wielding stances. In addition counter attacking was near impossible in edge-to-edge defenses even quick ones like poking and wide regular blows you'd throw if you weren't in clinching swords together were near impossible.
Now I experimented with flat blocking. I was amazed how many hits it took to show any visible damage and even the visible damage wasn't chips or anything that would slightly affect sword usage but simply scratches and ruining the sword's painted art a little bit. In addition it was very easy to "redirect the opponent's force around esp back at them". By this I mean not only did it take minimal effort to block blows with a single hand but I felt it was easy to do further movements such as moving the sword in a circular movement so that you won't feel any impact from the hit. Going hand in hand with this because its so easy to direct the force counter attacks such as hitting with the hilt, slamming the flat on their face, and quick pokes were so so damn easy its ridiculous and I even found myself redirecting blows enough that my friend was wide open to regular wide sword swings. Even with I don't use force direction movements, it was pretty easy to tilt my friend to trip or something simply because blocking with the flat sent physics motion that made him unbalanced, even dropping the machete a few times. Without redirection, I can still easily counter attack with the flap by hitting with the cross guard or flap directly in a brute force push. When you add in using the second hand, it becomes easier done than said to do even advance techniques.
So I am wondering. With all the proponents about how the debate is useless and flat blocking shouldn't be used as primary defense because the sword will get damaged anyway and edge to edge is much quicker and easier to catch a blow in time, my experiment makes me skeptical of the recent counterarguments to Clement's conclusion. Trust me it took a single machete swing to chip my scimitar heavily and it was much easier using the flat for defenses than edge to edge defenses. It makes me think there's a lot of truth to Clement's statement and the recent HEMA experts who argue against it such as Matt Easton are wrong!
Can anyone clarify? I must point out I don't practise HEMA much (though I've been attending a school for 3 years) and my sword was a generic one bought at Ebay for $35. So I'm not sure if the chipping was due to poor quality or my lack of practise in swordsmanship.
Zerothly, DO NOT SWING CHEAP SHARP SWORDS AT EACH OTHER. Something will break and you or your friend may be seriously injured or die. Again, DO NOT SWING CHEAP SHARP SWORDS AT EACH OTHER
Now that that's out of the way, about your actual question:
Firstly, you've basically asked this in the wrong subreddit. You'll get much better answers to this sort of technical question in r/wma. I seem to recall you actually already asked this there and got the answer, so I'm confused why you re-posted it here at all.
Secondly, what's most relevant is the evidence we have in historical sources. The opinions of modern 'swordfighting experts' are largely irrelevant compared to these, since none of us fight with swords. We have two particularly useful types of historical evidence here: fencing books and surviving artefacts.
Fencing books give contemporary technical information on the correct use of swords (or other weapons) in the opinions of their authors. Looking through a quick selection of these, we can notice that using the edge in parrying is often explicitly instructed. For some examples, consider:
In fairness, there are also a few plays given in fencing books which use flat parries "If he wants to draw after, catch the stroke on the flat on your Messer, so that the short edge is against him." (Johannes Lecküchner, Cgm 582, 1482). These are unusual and often make use of some specific trait of the weapon being used. The play I just cited is for the langes messer, which is a one handed sword with an extra guard on the side (known as a nagel). This extra guard protects the hand from the opponent's blade sliding down your flat and taking off your knuckles, which is otherwise a serious risk with this sort of flat parry.
This is generally supported by examining surviving swords. Edge damage is very common to see - I was at the Bunker Hill museum the other day, where they have a lovely late 18th century hanger that was used in the battle. It has two clear sets of nicks in the edge - one group closer to the hilt (likely from parrying) and the other group nearer the point (likely from having attacks parried). We also have plenty of original swords which show evidence of significant resharpening - the well known writhen-hilted sword in the Royal Armouries is a fine example. When you look closely at it, the edge has clearly been reground, probably more than once. High quality modern experiments in fencing with sharp swords suggest that even quite severe edge damage can be fairly easily fixed like this.
So that's the historical evidence considered - and it's pretty consistent and conclusive, at least for European fencing systems. But what about the risk of edge damage? Well, it turns out there's a simple explanation. Most fencing systems think about a sword blade as having two basic parts: from the hilt to the middle is the strong (sterck, forte); while from the middle to the point is the weak (swech, foible, debole). Generally speaking, the strong is used for blocking and the weak is used for striking - so damage to your edge from parrying will not make the slightest difference to the effectiveness of the part you might hit someone with.
Finally, I'd like to finish by reiterating the most important point: DO NOT SWING CHEAP SHARP SWORDS AT EACH OTHER.