Someone wanna explain to me in simple terms how it affects ballistics?
It doesn't.
Do you get slower velocities from losing pressure?
No. You don't loose pressure in the barrel. The bullet has already exited the barrel. A MB is added on to the length of the barrel and has a larger inside diameter than the barrel does and the bullet does not make any contact with the MB.
A MB works when the gasses that propel the bullet make contact with the surfaces of the holes in the MB and actually pull the barrel against the recoil.