There is a inset hole that takes a tiny spring with a small end piece attached that indexes on a specially milled section of the barrel shaft creating the required tension.
Had to do one a bit ago. It had lost the tiny end piece on the tiny spring. I ended up making one from a 4 p finish nail. [ didn't want to wait another 4 weeks for parts from the UK.]. Worked great. But now I can't get the thing to cock.... sigh... I'm down to opening up one of my other HW45's.