In ving tsun kung fu, the heel is our source of power. When we ground ourselves against incoming force, it's our heel that provides the power. Similarly when we kick, our two heels come into play - one to reinforce ourselves against the ground and one to strike the target.
The heel is the strongest part of the foot and is therefore our preferred striking surface. Sure there are styles that hit with the toes or the ball of the foot, and I don't mean to disparage those methods because they have some value, but this is not a zero sum game. There are many "right" ways to kick.
So when we land our kick, we are going to try and land the heel on the target; most often the opponent's knee.
This technique can be practiced on the ving tsun wooden dummy or the plum flower posts. I will suggest to kick 100 times with each leg for 30 days with a focus on hitting with the heel.
If you kick, and the kick pushes you back then your rooting was not strong enough and you should try to land a forceful kick that presses you into the ground.
If you kick and miss, and fall forward, then your balance was off when you kicked and you have increased risk during a fight.
Try to kick with power and stability. The 30 day practice challenge will help you explore and improve your kicking skill so it can become a reliable addition to your toolkit.