But I vaguely remember this is a myth

You have a fine memory!

Anyone know details?

Indeed. As with all bits of propaganda, there's always a small grain of truth. This myth arises out of an incident during the Invasion of Poland in which a squadron of Polish Cavalry managed to surprise a Battalion of German infantry marching in open order. They charged while in saddle and dispersed the German infantry, but were in turn scattered and turned back by the intervention of German reconnaissance cars that had arrived to help. This naturally evolved into the myth of brave Polish lancers dashing themselves futilely against Panzer Is and IIs - an outright falsehood.

In point of fact, the only significant mounted action actually occurred between two cavalry units; the German 1st Cavalry Brigade actually fought their counterparts saber to lance. They clashed with the 7th Polish Uhlans, whom at one point pushed on the headquarters of the Brigade, before being turned back.^1 The reality is that, like most horse-mounted formations, they practically always dismounted to fight on foot, and the Polish cavalry did so in a manner that is only to their credit.

In terms of battles against Panzer units, in the early phases of the invasion we know that a Polish Cavalry brigade, fighting dismounted and using Armored Trains and anti-tank guns, attempted to halt the advance of the 4.Panzer:

...the main Polish defensive line was in the scattered forests about 32km from the frontier. The most intense battle in this sector occured around the village of Mokra, which was held by the Wolynska Cavalry Brigade. Despite repeated attacks by the 4th Panzer Division during the day, the cavalry held its ground and inflicted significant losses...at least one of the German attacks was repulsed with the assistance of [an Armoured train]...

If the defense of Mokra was proof of excellent training and morale of [the Polish Cavalry] it was was also evidence of its shortcomings in contemporary conflict. Casaulties were so high that the brigade was forced to withdraw that evening.^2

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1: Page 8, Axis Cavalry in World War II by Fowler, Jeffery T.

2: Page 47-8, Poland 1939: The Birth of Blitzkrieg, Zaloga, Steven J.