Your ankles are your shock absorbers on your horse, so they need to be soft and relaxed. If they are forced down that is just as bad as if they are up and you are using your ankle to brace in the stirrups. You want your heels down and lower leg soft and relaxed yet engaged. It is simply impossible to grip enough to stay on a horse - you stay on by keeping your center of gravity over your horse's center of gravity.
If you look at pictures of where a horse's center of gravity is and line it up with the rider it makes a lot of sense.
Heels down is about soft, not forced, to allow your body to to balance with the horse and move the horse and transfer the shock of movement through the ankle. Watch a jumper on landing or dressage rider when doing a test - their ankles take the "bounce" - most people think heels down is correct, when rather ankles soft and stirrup on ball of foot is correct which due to physics and leverage means your heel will be down but able to flex with movement of the horse.
Oh and when I say watch a jumper on landing - I mean any decent rider haha and landing bc i've seen riders have their legs slip back (even gp riders) but on landing their leg is under them and they flex through their ankles to absorb the shock of landing the way the horse's fetlocks flex. If you tried gripping, you'd slam the horse in the back. Hope that helps!