The Horse Stance (also called Horse Riding Stance) is a posture frequently used in the Asian martial arts, as well as in Tai Chi and Taoist Qigong. The name derives from the position/pose itself, which looks like that of riding a horse.
In fact, it’s very much like performing a squat (with the feet shoulders’ width apart, or farther depending on the style), and the pose is commonly used to exercise the body’s core and pelvic floor, and to strengthen the legs and back. It’s seen as a body grounding and stabilizing exercise.
As for the application with the Yoni Egg inserted, it means doing Pelvic Floor Exercises, Vaginal Weightlifting, or dynamic inner movement exercises in Horse Stance position.
In the Taoist version of the Horse Stance Exercise, you will typically also clench the fists while doing the pose and combine it with Ovarian Breathing.