In Bagua, this is called by some the "bagua foot." It's a hooking step that sets you up to apply pressure on your opponent's calf and cause them to lose their balance.
In the second photo, Colin presses forward with his shin, applying pressure against Kim's calf. This causes her left shoulder to move backward and to the side.
This is a great internal technique. Remember, the idea of internal kung-fu is to unbalance your opponent and take advantage of the window of opportunity that becomes available when they lose their balance.
You remain centered -- your opponent loses their balance. This is your goal mentally and physically when you are attacked.
In practicing Bagua, the ko bu step (into a pidgeon-toe stance or a T stance) can be used in this technique, and this is one of the reasons it's practiced so much in Bagua. Footwork is important in all martial arts, and this is one of Bagua's most important stepping techniques.
Remember to sink your weight through this technique.
When you hook your foot behind your opponent's ankle, it also sets you up for a "split" technique where you kick the opponent's foot while exerting backward force against the opponent's torso. More about that one in another post.