How do you hit a low net clearance shot with a lot of topspin that is also deep?
Swingpath should be slightly low to high but still not much, so you create some spin but also alot of pace and direct energy.
Racquet face should be closed slightly to create a lower trajectory (bit lower over the net).
And you should use alot of windshield wiper movement to create alot of spin (forearm pronation).
Use just the right combination of these 3 factors so you have low clearance as you wish and enough pace/spin combo so that you can swing with 100% RHS and the ball won't go long but dip into the court.
Basically the secret is you close your racquet face more so the trajectory of the ball will be lower and you can brush even more upward and since ur face is slightly closed the ball will still go fairly low even when brushing upward, and then add even more spin by windshield wiper = a low trajectory ball with alot of spin.
But you reach a certain point where if you close it more and brush it up more that you lose too much pace and the ball won't have enough pace behind it, thats why I said close ur racquet slightly and brush upwards but still fairly towards the target so not extreme upward swing.
As you can see the more you close ur racquet face the more upward you need to swing to produce the same ball trajectory (height), otherwise the ball will go down into the ground, but the more upward you swing the more spin you will get.
So you need to close it just enough so you can stil get good pace and a nice low trajectory and you will get alot more spin than if your racquet face was perpendicular.
But the more you close ur face the more upwards you need to swing and the more pace you lose.
And also you reach a point where you close it so much that no matter how much upward you swing you wont even clear the net, this pic is just for reference, if you actually had it closed like the 3rd pic you would hit the ground.
If your racquet face would be perpendicualr and you swung like 2nd pic you would get a higher arc over the net, but with ur racquet face closed enough and the swingpath being the same you get a lower net clearance with the same if not bigger spin, so thats the secret you are looking for.