Originally Posted by Wegner
Just move your head and your upper body to start in one direction.
The outside foot will slide underneath the body, which now, unbalanced, will move on its own.
Lead with your racquet’s edges, top edge for topspin, bottom edge for slice, and you’ll have more control.
These are more or less correct.
Only things to add are that for many beginners, the outside foot does not automatically slide out towards the ball, as the natural tendency is to crossover with the inside foot (in real life, you usually want to turn and step across if something interesting is on one side). So this might have to be encouraged a little by a coach if it does not happen automatically.
For the second point, the terminology "lead" is indeed correct. Perhaps it is simpler to say "keep the face a little closed for top spin, a little open for slice," which is the same as the above. Important thing is that leading with one edge does not mean that contact will necessarily be towards that edge of the strings or the opposite edge, or in the center, but will depend on other things. A steeper lead will favor contact with the lower edge of the strings, a shallower lead more with the center.
Related to that, there is a subtlety that for topspin, the actual contact position is often not closed. The racket face is often vertical at contact. This is seen in Fed and Nadal forehands. It is difficult to see how this can be reconciled with a closed face or leading with the top edge. It seems that there is supination or opening of the arm on the forward swing till impact, followed by pronation during the dwell time and after. Otherwise, it is impossible to reconcile vertical face at impact with supposed closed face or leading with the top edge. I think the vertical to closed transition is what gives the solidity at impact. Instead of the swing path being a single arc over the ball, it is more like an arc coming up from under the ball and then closing over it at the end of the dwell time.