Hi.
The idea, is that all moves require the body to respond a certain way.
It goes to where you want your weight, I have seen Katas where they show the heel up and they will call it the old monk or old man Kata?
Does this make sense? Do old people walk spryly?
As you are walking the step is an act of falling forward and gravity taking over until the next step intervens and keeps you upright.
In the military it is the heal that makes contact, heel down while riding a horse helps you stay in the saddle.
Is the heel up or down, we can go on and on, each Kata has a way of teaching and one Kata will teach one way and the other will teach a different idea, the reason for this is that each are for your information.
The time for it to be used, while moving or sparring or fighting is based on your training or instinct, after all we have been walking a long time.
Good teachers talk about training and how you revert in time of stress. I believe that is the main issue, when the time comes to use, if you have been trained or train alot, based on your observation, (gut felling) you will use the correct or your trained tech at the moment it is needed.
Forms, Katas, are the essence of the martial arts, if it is not in your tool kit, you need to go and get it. Talk to your teacher, read, discuss, ask questions, watch others when making the same move, is it fluid or stilted?
If a runner is on the balls of his feet while moving forward, when he stops running he walks with the heels for support, he is tired, if you need support you need the heel down, if not it can be up. You will see that in horses when they jump the heel takes the first contact.
Sliding the foot, trying not to move it off the ground, not far from always being there for support.
Moving quickly, the ball of the foot will be primary, if the muscle is there to help, if not, it is the heel and skeletal structure that does the job.
Last but not least, in the Yin and Yang thought process it is a way of combing the two that gets the job done.
Regards, Gary