In my opinion, if you are using 4000 grams springs, ( per pair), then you will have to tight the noose/strap very tight in order to not slip out. Thus, you cut off all blood supply and the penis goes cold. I would suggest that you get yourself springs with lesser force, preferably 2000 grams or less. In that way, you can have the noose/strap not so tight and can have longer sessions, beacuse you will be able to - compress the extender with one hand, and jelq some fresh blood into the glans. There is also a correlation between how tight the noose/strap will have to be, and the length of the compressed extender vs the length of your stretched flaccid penis while strapping in. If the extender is too long, the noose/strap will pull to much forward, and try to pull «through» the corona/edge of the glans, ( resulting in slipping out), instead of just securing the penis to the extender and let the extender do the pulling/stretching foreward.
Well, that’s just my two cents. Good luck.