I would agree that technique is almost everything. My wife has very little strength in her hands so I can let her grab and squeeze both my balls at once as hard as she can with only minor discomfort. However she has come up with more effective methods. One is the two fingers and thumb as has been described but her favourite is to form a circle with thumb and forefinger. She will grab the bag to isolate one ball and then squeeze fingers and the base of her thumb. Since it is the base of her thumb it get more leverage and can exert much more pressure. The thumb also presses down on the top of the testicle which is more sensitive and she can pull down for even more pain.

The above grab makes it very difficult for the testicle to slip out and without much effort on her part she has me begging her. Often she will grab and hold one this way and whisper in my ear "no escape for your testicle" "no hope for you".