The first 36 seconds of this YouTube video shows a Coffman engine starter cartridge being inserted into the firing device. The aircraft is a WW2 U.S. Navy “wildcat” fighter.
A comment by “Conrad” explains the functioning in some detail and is worth repeating here:
“The Coffman starter uses a specially made 4 gauge paper shell with an electric primer. It is filled with .25” and .187" diameter cordite pellets for slow burning powder. The shell fires into a starter assembly on the accessory case of the engine, same position as an electric starter. It DOES NOT fire directly into a cylinder of the engine. The gasses force a piston inside the starter assembly forward towards the engine collapsing spiral gears on top of each other converting it into a circular motion. This engages the starter dog and rotates the starter gear. After the piston reaches the end of its travel a valve released the residual pressure and a die spring resets the whole process."