In this case the depth of your shovel blade would have to sink to below the top “duff”, which is anything above the soil that cannot hold moisture.
This may seem more obvious than it is, but with the great variety of terrain you will come across, you will find there are definitely many ways you can skin the same cat. If the ground is rocky I find the best strategy is to wriggle the blade in, maneuvering it around the stones as you get it deeper into the ground. If the ground is soft enough, slamming it in will definitely shave off those valuable micro-seconds. Use the kicker to give you added pressure when necessary. If using the kicker let yourself get creative, such as by using your right foot on the right kicker, or left, or left foot on the left kicker, or right. You do not ALWAYS have to get to the plantable spot with your right foot ready for the right kicker. When walking around all that slash you may simply find your left foot closer once you get there. Get used to thinking ambidextrously. It will be more ergonomic on your body, and why waste the time reshuffling yourself into your “standard” position (such as right foot forward)? Other times I’ve seen very fast planters who say they never use their kicker (they actually get them cut off to reduce weight and so that they do not get caught in roots when pulling the blade out of the ground), but always have two hands on the handle when inserting the shovel. With a tree already held in one hand, they put the weight of their shoulders along straight arms over top of the handle and provide sufficient pressure that way. Be creative and let yourself experiment. The more ways you learn how to skin the same cat and the more practice you will have with each technique, the more diversely you will respond to each particular situation. Don’t let yourself fall into a rut or some thoughtless routine. This is a thinking game.