The official SOE catalog referred to this as a “short length, silent, murder weapon”. The sleeve gun was a variant of the better-known Welrod pistol; it was a simple suppressor tube with a single .32 ACP caliber round loaded into the breech before concealing up the user’s sleeve. A hole at the rear of the gun allowed a lanyard to be attached. The lanyard could be looped around the elbow, keeping the sleeve gun concealed until it was needed. With a rubber band providing tension, the Sleeve Gun would disappear back up the user’s sleeve as soon as they let go of it after firing.

Rather than a conventional trigger there was a simple knurled switch that was pushed forward and then back again with the thumb to fire the round.

https://www.thetruthaboutguns.com/the-guns-of-welwyn-five-incredible-weapons-of-the-british-special-operations-executive/