A booster also known as a linear decoupler or a Neilson device is a part added to most hand gun suppressors so the gun will still function with the added weight of the suppressor on the end of the barrel.
The booster is a piston that uses some of the gas from the fired round to basically push the suppressor away from the gun as it is fired and at the same time pushes back on the barrel so it can unlock and the gun can cycle.
It is late and I'm tired so look here for more explanation... http://en.wikipedia.org/wiki/Muzzle_booster
The boosters are the bits with the springs around them to the right of the suppressor cans in this picture. Each company makes them a little different but they all do pretty much the same thing.