The BH can make a real good topspin with pretty much a short stroke. It is much easier and natural to make a short area whip with BH.
Next Level has seen some of my BH topspin and it has also gotten much better since he last saw me. Next Level know the BH and how to teach it too, he also has a lot of pro friends who can teach it.
How one does the shot will depend on the incoming ball and what you want to do.
It comes down to a fast bat at impact and how you grip the bat at impact. You get a fast bat by doing something to start kinetic energy, then amplify it with lower arm, wrist, and fingers. The hitting upper arm and elbow will get to the position needed and be stopped, then stable/loose. This stopping while loose transfers the power to the lower arm if the elbow is also loose and used as a lever. Once power goes there, one engages lower arm, then wrist, then fingers (tighten only right at impact for additional power/spin or keep looser to eat through existing spin on ball)
The shot can be done from anywhere... but it is a lot easier to control the ball and put on your own topspin when you take the ball off the bounce.
It can also help to impact the side of ball a little to help over ride heavy incoming topspin. (easier to do a step away from table vs a medium or fast loop)
The few things I can say to visualize or improve the road to learning a good BH topspin (if one is hitting flat) is to stay loose every where, not go for wide sweeping stroke, lift the ball a little more, and not hit the ball as direct.
That is a lot of general talk and one needs to stay loose (easier said than done) and discover the feel of the impact for themselves.
One exercise coaches do to develop the feel for the ball on this is for partner and player to crowd the BH corner of table and do off the bounce BH to each other RIGHT OFF THE BOUNCE... like only centimeters from the bounce... keep wrist and arm loose, go for short stroke... borrow speed and spin of the ball... use a very short stroke (one gets the importance of a short stroke here at first, like not even a foot stroke)… once timing off the bounce is improved to be very consistent, add more power by stronger use of lower arm and more firm tightening of grip at impact... progressively tighten grip more at impact and learn to allow ball to come up just a bit more for this powerful short are BH topspin.
Doing this exercise with a player who has consistent and strong BH will do wonders... even if it is just 5 minutes a few times a week. I see many kids growing touch for this shot this way. Even in Korea, where the BH is well know for being very flat and coaches teaching a flat BH, the coaches developing the pro-style trained kids get it... and show the kids how to develop touch, spin, control then power (spin and speed).
If one hears that the BH can be spinny and controllable with a tiny stroke, one would think someone was off their rocker. Yet, it is true, a BH can be powerful without a long stroke. Even my longest counter topspin BH stroke away from the table is no more than 2 feet long finishing in front of my chin and chest. Maybe the opening loop vs underspin requires a longer stroke (you are getting down and exploding more), but the OP is (I think) asking about a spinny higher arching BH topspin rally shot or counter BH Topspin.