the at cam has a flat spot in the pin that goes through the frame, when the pump handle is fully forward the flat is facing forward and the trigger can be pulled far enough to release the sear. as you pull the pump handle back the cam causes the pin to rotate and the trigger cannot go as far back.
there for as your holding the trigger when you pump it the at pin pushes the trigger forward far enough to cock the gun, then as you are pulling it forward it releases at whatever point your hammer is set at. this is where timing the AT comes in.
lmk if thats too confusing
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