If you can't tell what he's trying to do, it's this.
In addition to now being an excellent stander (while holding onto something sturdy), Jack's latest trick is that he has learned how to pivot. Sit him on his bum in the middle of a pile of toys and he can turn himself in circles to reach them all. He's also getting good crawling practice each day as I stack blocks or cups just out of his reach and he has to work hard to get close enough to crash them down then bursts into fits of laughter at how clever he is. I have no doubt that the day we are moving when we need him to hold still will be the day he decides to become mobile.