Some of the older types og grease gun have a rod that pulls the plunger back and some have a chain. When the body of the gun is full of grease you hitch the rod or chain into the keyhole slot to hold it there temporarily. The halves of the gun are screwed back together and then you can release the chain or rod and stuff the chain or push the rod back into the gun body. The chain type fit with a bayonette fitting. The spring loaded plunger then keeps the grease pressing lightly on the pump mechanism to stop airlocks developing. At work, we used a couple of AEROSHELL greases, 7 and 22 for lubricating aircraft parts. When a gun was empty we used to ask an apprentice whether he knew how to refill the grease gun. Sensible ones asked how and were shown, the bigheads who thought they knew everything were heading for a messy accident.
The cartridge type are nicer to use and load like a mastic gun.