One leg hamstring curls
Coach is doing one leg hamstring curls here. This cable exercise is great to focus on each leg as you curl up the leg as shown. Also, you can really squeeze the hamstring on each leg to get a good pump to help the growth. Finally, one leg hamstring curls allow you to do drop sets to lower the weight and get more curls in for pain and strength gains!
And yup that is his jockstrap on!