What exactly happens when RF connectors are over or under torqued? How does it affect the signal transmission?
Under-tightening it can leave a gap or make a spotty connection. Over-tightening can crush the ground or the center conductor. Both of these can degrade the signal, sometimes rather significantly.
ETA: Some people are a bit anal about getting the torque exactly right and they even sell calibrated torque wrenches for this sort of thing. In my experience, you can hand tighten them and as long as you have a decent feel for what you are doing they’ll be fine. I’ve seen some pretty heavy-handed people overtighten them though, and a torque wrench does prevent that.
Hams use a lot of PL-259 coax connectors. We always say the PL means use pliers to tighten.
when the knurls rip the skin off your fingers then it’s too tight.
there are wrench and finger connectors.
yeah gaps are bad. over tightening can also distort the socket producing gaps.