They cannot shoot or be shot at. Along with that you cannot advance on that player. This does not mean you cannot move. It is a common feature on guns today. It increases the rate at which paintballs fall into the breech.
Most guns come with power feed on the guns when purchased.
Some guns will never have them like the Angel. Regulates the pressure of gas flowing through it. Some regulators are preset and some are adjustable. A pin which can be removed quickly to speed disassembly of a player's marker. This lets a player disconnect his gun from the air-source quickly.