it's bloody obvious jim if you think about it.

i'll try and explain it, but it's a bit tricky without pictures.

a cam lobe is effectively egg shaped right? the pointy 'top' of the egg (that you slice off and dip your soldiers in) is the bit that operates the valve (via whatever kind of follower).

the opposite side to that is the more rounded bit that is basically just part of a circle (the full diameter of which would give you what's called the base circle).

to get more lift from a regrind, you obviously cannot add material to the pointy bit of the egg, so you have to make the base circle smaller so that the pointy bit is now a larger proportion of the whole egg, thus becoming a longer lever to create more lift.

make sense?