Though this is not a perfect front hip circle example, notice how the gymnast raises her center of mass forward to begin falling down without changing her body position. Then, when gravity starts to accelerate her fall, she pikes slightly to add some extra circular momentum while keeping pressure on the legs to allow them from moving away from the bar even at the moment that she briefly lets go of the bar to shift her grip around to prepare the hands arrival to a full support.
Still Frame 1: Fall forward has began with the thighs pressed against the bar.
Still Frame 2: Grip is shifting around while the legs maintain bar contact
Still Frame 3: Throughout the skill, the goal is to continue pressing the legs against the bar instead of allowing them to move away.
Check once more some of the skill technical details.