Opening your hips.. basically when you flex your leg up under neath you *so your toes touch your bum) and reach behind to grab your ankle, that pull is you coaxing your hip to "open"
You can do this in the saddle or standing up as long as you hold onto something to be safe.
This allows you to really sink your seat bones into the saddle versus sitting all perchy.
However, it could also be due to your saddle and the way it is built.
Now for my critique
I love the soft straight back, brilliant! Ear shoulder hip heel line is pretty good... though it looks as though you're clamping or tight in your knee. See the swing instead of the drape? You need to get your weight down your leg without any clamping going on.. Think of your leg stretching down with your toes up, instead of jamming heels down. Pull your knees off of the saddle, so they are barely touching.
Very nice improvement :)
Hands are slightly too low, see the break in bit to forearm line? It should be nice and straight. Roll your shoulders back and open up your chest more.
"Strength is the ability to use a muscle without tension"