In order for me to extend the shaft, not the foreskin AND to facilitate monitoring on the Glans color/temperature ( signs of circulation), I chose to always retract my foreskin.
So far, the preceding post do not address the method of fastening into the extender (noose, strap, chamber/vacuum). Depending on method used, the effect upon circulation ( either in glans or foreskin) is effected.
My gut feeling is one wants to be able to keep an eye on conditions when starting out.
If you have a very tight foreskin/problem retracting it, you might want to do some exercises to loosen that up, first.