Stand in your stirrups, with both long dressage length leathers, and up short in 2 point. Heels come up because the rider is gripping/holding with their knees and thighs, thus lightening the seat. If your horse decided to buck, or tripped, you'd be over the front before you could say 'bugger!'!
Getting up in your stirrups will help you find your centre of balance, if your heels come up you'll fall off, so you learn pretty darn quickly where to keep your lower leg. It's also physically impossible to grip your knee and thigh in this position.
When you return to having your seat in the saddle again, you will have a far more stable seat and your horse will work better for it.