Hanging by the balls is a great way of exercising your tummy muscles. It is, for me, at any rate. I managed 90 seconds today. There is always chafing afterwards, so I slap some moisturizer on.
I know I could do more than 90 seconds, were it not for the strain on my stomach muscles; and that is despite being fit with a four-pack at least.
As well as the stretch, one needs to stretch right up to grab the rope at the end, in order to pull my self up and get my feet on the ground.