The difference between closed bolt and open bolt is the position of the nozzle when the system it is at idle (waiting to be fired). With an open bolt system the nozzle is back and a closed bolt the nozzle is forward. When you fire an open bolt system the nozzle is driven forward so that it chambers the BB and then fires at the end of its travel before retracting back to the rear position. When you fire a closed bolt system the system will fire first, then cycles the nozzle back allowing a BB to enter the chamber before the nozzle returns forward to push it into the bucking.A closed bolt system is more forgiving of the hop-up system since the BB is in the chamber and everything has stopped moving before the system fires. An open bolt system is more critical of the hop-up system since it is pushing the BB into the chamber and firing in the same motion so the BB never actually stops. This is why open bolt systems typically require a bit more tuning of the barrel group than closed bolt systems in order to optimize accuracy.