Ephraim has it 100% correct. Number one rule is to keep your balls inside the cup and the cup tight to your groin.
I typically wear boxer brief underwear with a pouch and a 3” athletic supporter with knit pouch (traditional jockstrap) and shorts on top. I use hard contoured athletic cups, often referred to as a ‘banana cup’. Five minutes before Krav Maga class starts, I find a quiet corner, turn my back, and insert the cup between my underwear and jockstrap. The boxer briefs protect inner legs from cup rash and wick away moisture.
I agree, you should check your cup regularly by slightly squeezing on the sides to check for cracks before wearing cupthe cup. I also recommend cleaning the cup with antibacterial soap in the sink and drying ait with a paper towel weekly to eliminate bacterial growth.
My only other added advice is to check and adjust cup as needed in training. If you are doing side kicks, or a lot of high front kicks, your cup can move. If a MLB baseball player can adjust his cup after every pitch, a Krav Maga student should have the confidence to check their cup 4-6 times in a 90 minute class.
Stay safe