I've played for over 100 hours of this game since first release. I still don't understand what this does :

Does it steal 50% of my food to produce defense? (in which case, most stupid law ever)

Does it increase food production by 50% with some other effect on defense?

I don't understand. Sometimes it's also -X% food to defense, weird.

Manpower the amount of sort of 'armies in waiting' you have. You need them fill ships, assault planets etc

Normally manpower is added to this pool at the rate of 10% of your food production. You'll actually see this in your food generation. 10% of your food goes to your manpower pool. At least until it fills up.

I'm pretty sure it sacrifies population growth for manpower. This is actualy a useful law if you're relaly low on manpower to fill ships and such but your empire ismostly 'full' with respect to population.