This simple little gadget has got me out of trouble twice since I acquired it recently. It’s used when you want to change an old gear cluster or to service the bearings in a derailleur hub from the 70s or thereabouts. Check first that you have this type of freewheel, newer ones (e.g. some 5 and most 6,7,8 speeds ) use splines inside the freewheel body To use it, you remove the quick release wheel from the bike, then the skewer from the wheel axle.
Fit the prongs into the matching slots on the freewheel body and refit the skewer, tightening it onto the tool. Then put the tool into a vice, closing the jaws on the tool flats and turn the wheel rim as if it was a steering wheel ( anti-clockwise, I think ) —- Voila !
The freewheel and cluster come off as a unit – don’t fiddle with the little round holes unless you have watchmaking skills, all the pawls and freewheel bearings are in there.. !
Just remember to loosen the skewer when the freewheel first lets go or you will snap the skewer.
I know this from my early experience !