Hi! This seems like a strange drill because it involves letting go of the bar, but IT WORKS because it gives the gymnast the "feel" of the front hip circle ... as in the momentum required for the skill.
From a front support on the low bar, have the gymnast press down on the bar so her thighs rise. She should fall forward, let go of the bar quickly with both hands, and immediately grab the back of her thighs, right above the back of her knees. (Her legs will need to be tucked after the initial fall forward). She will circle around the bar to the top, and her arms/shoulders/hands will end up over the top of the bar. She will need to re-grasp quickly.
Sometimes, girls are scared to let go of the bar, so I recommend spotting this drill at first. Once the gymnast understands where she must grab her legs to complete the circle, she will feel comfortable doing it! And once she gets this drill down, she will be much closer to completing a regular front hip circle, since understanding the momentum is so important in mastering this skill.
Hope this helps!