It can depend on whose definition you read but as I understand it (and YMMV), heeling a handgun is essentially gripping the pistol to low on the grip. This means the web of your hand isn't high on the grip where it is pressed against the beavertail so the pistol can rock back with the muzzle climb and recoil (backwards energy) and lift the muzzle (and front sight) up. IOW, your low grip on the pistol results in you shooting higher than your aiming point. This description is also obviously made with a semi-automatic in mind although the process could affect shooting a fevolver to some degree too.

I didn't see many people doing that so it never came up a lot in training (all with semi-autos for duty guns) although it's a basic part of teaching somebody how to hold a pistol for shooting. It's also something that tends to happen more if you're only shooting with one hand as opposed to using 2 hands.

Since OT brought it up, I'll defer to him on the nuances of it. His agency may have had a different explanation and training sequence. Hopefully this helps some.