I recently read alot about medival knights and roman cataphracts, and on all the pictures they carry long lances.
But aren't lances (on horseback) completely useless, because when you charge a enemy with said lance and penetrate his body what are you going to do? Let go of the lance? Try to get his body of? If you just leave the lance after one hit why even bother?
I apologise for not being great at reddit links etc. but I googled your question and found a much older question in this same subreddit that seems to basically answer your question
If I had to summarise, it would break down into possibly 3 pieces:
Just the threat of a cavalry charge was often enough to be effective. Especially if an infantry unit was already wavering (which is often when cavalry would be deployed to deal a killing blow to their resolve) soldiers standing near the site of an impending charge would know they had almost no chance of survival and would try and get out of the way, which would expose the second line to the same impending doom and this could create a cascade effect that would often turn units into full retreat sometimes even before the first lance made contact with anyone
Charges were just that. Charges. They were not meant to be prolonged engagements. Cavalry units would often try just to hit an exposed flank and quickly run away soon afterwards. They could deal massive amount of both physical and psychological damage by acting in this way. Some horses my get trapped slightly deeper in the infantry ranks then desired, at which point they might fight their way out with their swords. It was also mentioned that lances would break fairly often, but if the bearer of that lance could retreat successfully it was sometimes the case that they would actually resupply with a spare one which they would use for subsequent charges as required
There were also some vague allusions to some sort of technique for maintaining grip on a lance during a charge in the thread I linked to. Something about loosening your grip and allowing the lance to rotate in your hand. (Essentially, push the lance through the target, and by extension push the target towards the ground, and then use the targets weight to pull the end of the lance down and eventually behind your horse as you ride, at which point, they may be dragged off the end of your lance by friction with the ground/their own weight, allowing you to bring it back up for another charge. It’s also possible that you may be inflicting lethal or near lethal damage on some of your opponents even with glancing blows)
You might also be interested in this answer by /u/hergrim describing a hypothetical knightly cavalry action. It's a fantastically entertaining read as well!