Pretend like you are holding a huge steering wheel. Stand on your right leg, bend your left knee up to the left elbow while you mock a left turn on that steering wheel. Then take that leg behind the standing leg, returning the wheel straight.
Repeat this move and think about the last rib on the rib cage meeting that same side hip in a pinch or squeeze.
You'll find this move makes the entire hip, stomach and glute area work very hard. To challenge it further, do this move without allowing the left foot to touch back behind. That is very tough to master.
You might even start by using a chair or counter top to hang onto at first. Do as many as you can on one side for 60 seconds. Then, switch to the other side.