Usually resistance welders are equipped with a simple pneumatic circuit providing a single force on electrodes.
Some welders are equipped with pneumatic circuit allowing the electrodes to approach the piece to be welded with a lower force. This avoid damages on the piece. It is useful in projection welding.
Other welders are equipped with a more complex pneumatic circuit allowing also to increase the electrode force during or after the welding. This function is called FORGE. It is used mainly in projection welding and high quality alluminium spot welding.