Hi Matt,
I took one look at your vid vs underspin. Before I get into any analysis, you have three basic topspin shots and a couple of flat hit options vs this ball. What kind of shot you decide to play will depend on the incoming ball and your ability to get there to give yourself options.
In this simple training situation, the ball is pretty much coming to your FH prime zone and you have every option available to you.
Before I can get into what you are doing and what I think you are trying to do, you need an understanding of your options and to basically perform them.
Your topspin options...
- Power Loop (fast topspin) You open the blade face only a little more, make very solid contact a little off center (above) axis of ball, drive through the ball with only a little catch and throw feeling... swing is very much forward. result is a fast ball with good spin. This requires an explosive stroke and firm impact. You are tightening very firm immediately at impact. This can be a point ending shot or get you a weak return to kill... or a fast block to put you under pressure or lose the point.
- Slow Loop (heavy topspin) You squat down with legs and hips... get top of hips even with or even below top of table, explode UPWARDS... more upwards vs heavier underspin... less upwards vs weaker underspin... stay loose throughout the stroke and impact... firm a little at impact, not a lot. Open blade face.. more vs heavier underspin. Much brush contact, very little solid contact with ball. This can be very devastating... timing of ball is so slow it wrecks timing of many opponents, especially those who like to play fast all the time and have trained only vs fast balls.
Medium Loop - very versitile... you are doing something in between the two above... you are going 50/50 on solid and brush impact. You do not need a full stroke or super fast bat speed. Use medium grip pressure at impact. Open blade a little, balance between forward stroke and lifting stroke. This is your basic pressure shot that is safe.
Fast Hit - this is a shot with almost no topspin, but it is very fast and a "wierd" ball for many to handle. You open blade face, swing forward, impact ball on center of axis, finish slightly higher against stronger underspin, more forward vs weaker underspin. FIRM grip immediately at impact.
Medium Hit - like above, but keep grip loose and maybe lift a tiny more... pretty much same stroke, but impact grip is loose. You are killing spin and sending a slower, but still kind of fast ball with no spin... easier for you to control placement.
Now you understand your options... what were you trying to do?
From my quick look at the vid, your result was a slow-ish ball and it didn't look like it was very loaded with spin. Your feet position were too much BH oriented... not an open stance at all. Make your right foot AT LEAST another 15cm if not 30 cm more to the rear... later even more if you want more FH power and use of hips. You swung mostly forward and tried to fine brush the ball... very little solid impact. I cannot tell your grip in a vid, but maybe look for clues.
If you are trying to make a fast loop... OPEN YOUR STANCE, open blade some more, swing forward, MAKE MORE SOLID IMPACT - not a ton more, but more. Impact ball a little above center behind the ball. Try to let ball come up, but not above top of bounce. Closer you impact ball to bounce, the less you need to adjust bat angle.
If you want a medium loop... OPEN YOUR STANCE, swing less strong power, use 50/50 for amount of brush/solid impact (you will still need to impact ball more solid than in your vid) use 50/50 on forward/upward stroke, do not death grip the bat at impact
If you want a slow, heavy loop... OPEN YOUR STANCE MORE, get down, swing upwards, fine brush the ball like you are doing in the vid, swing almost straight up vs heavy underspin, a little more forward vs weak underspin, keep grip pretty much loose even at impact... firm it up later when you get timing down.
If you want a fast hit... do like the power loop, but strike behind ball at center of axis.
If you want a medium hit... do like medium loop, but strike behind ball center of axis and keep grip loose.
THERE. You now have your available options, how to basically do them... and a quick analysis of what you were actually doing compared to what it looked like you were trying to do.
That will be a $150 USD invoice payable by credit card... that would be... if I was actually a working pro coach in a club... which I am not. Do not pay, I offer the analysis and advice freely. That is why I would never cut it as a pro coach. I have a natural tendency to help people for free too much... and it pisses of the coaches in the clubs I play in.