It's the idea that with the same bullet at the same speed, one powder might give less 'felt recoil' than another.

For example, in 357 you can reach 1,000 fps with a 158 JHP using Bullseye or 2400.

Mathematical recoil formulas would have it that the lower charge weight of Bullseye would give less 'free recoil' than the 2400 load.

OTOH many experienced shooters would feel less recoil with the 2400 load because the bullet accelerates more slowly and gradually than it does with Bullseye. 2400 and other slow powders seem to 'shoot softer' for any given speed.

Of course it's all perceived between the ears, so what works for one may not work for another.