I think mildot is correct.
Weight cannot be 'in your heels' because there is nothing under your heel. Your heel, like your knee, is a hinge. Unlike your knee, the heel has no possibility of actually supporting weight, since there is nothing underneath it. 'Weight in your heels' is mental imagery, not physical fact. None of your weight is supported by your heel, because there is nothing under your heel to stop the effect of gravity.
If the stirrup is on the ball of the foot, then the heel is a hinge. Weight can flow thru your heel and have the stirrup support the ball of your foot. Hinges can resist the flow if the hinge is pressed against the horse (fixing the hinge in space), or if we use muscle tension to transfer the weight somewhere else.
If you fix the hinge against the horse by pressing your knee into the horse, then you have no control on how the hinge works. It will affect your riding by the combination of gravity & the horse's movement. You lose the hinge and create a pivot point.
However, one of the big advantages to a hinge is that we CAN apply muscle tension to it, and absorb shock by slowing the motion of the hinge. I don't jump, but I've read that an advantage to jumping with the stirrup on the ball of the foot is that it allows your ankle to flex. Muscle tension applied to the hinge slows the motion, and absorbs some of the shock.
When instructors say, "Weight in your heels", I think what they really mean is to relax the muscles around the hinge, and allow gravity to pull your heel down as far as your body allows it to go. This also transfers all the weight possible into your stirrups, making them more secure on your foot. You can ram your feet home in the stirrup and remove the hinge, but that is considered bad riding in English riding - unless you're riding polo or steeplechase.
"...You need to allow your heels to be your anchors and by allowing them to do this, you must permit your bodies natural weight to flow...that way, your ankles can relax and flex and act as shock absorbers.
Then, your heels will be allowed to do their job - which is anchor you in your tack. Open up your knees, do not grip or pinch..the moment you do that, you block that weight flow from occuring. Opening up your knees and allowing that weight to flow down..."
As a mental picture, weight going into your heels is what it FEELS like you are doing. As physical fact, your weight is either carried in your seat, your thighs, your knee (bad) or the ball of your foot. You heel cannot carry your weight because it has no support. However, any gripping with your knee will block the flow of weight, and any tension in your calf pressing down will lift the hinge, bringing your heel up and losing the function of your heel as a hinge.
Polo & steeplechasing doesn't involve vertical jumping, since steeplechase jockeys jump in a fairly flat style. They can ram their feet home to reduce the chance of losing their stirrup, but they lose the hinge. That is OK for what they do. In that case, the weight really is in your heel (or just in front of it).
When weight goes through your ankle, it must end at the stirrup.