This shot is obviously a bit of a controversial shot. Lets talk about when and how to hit it and then we can talk a little about the ethics.
This shot is one of those shots that should be used very sparingly and as a surprise or it will cost you more points than you will win off it. The problem with all potentially point ending shots is that you can easily get carried away and overuse them as a result of winning a couple of cheap points off it. The bigger problem is when you start earning some points from these shots and you start overusing them is how to get back to finding the balance to hit them the right amount of times.
The shot itself is pretty simple as I think it is most effectively used when you are a right hander (just reverse for a left hander) and you are on the Ad side of the court at net and the opponent you will be hitting it to is on the deuce side and you will be aiming at the right center of his body. The reason for that is that they have to move to their left to get out the way and then back to the right and forward which will hopefully give them the longest path to the ball. All of this will be done while they are trying to avoid being hit and trying to keep their balance. If the opponent avoids the ball it needs to end up tight to the side screen of opponents deuce side but preferably not hit it as that might slow the ball down and give them a chance to come back at you. If the ball crosses the net line before the second bounce you lose the point.
What you always need to remember is this shot is hit off a very short lob or it will not be effective and you will lose most points. It is also helpful if you already have the opponent off balance. So you pretty much need a setup shot to deliver the big shot. It is hit like a forehand drive ( swing volley) and lots of topspin can make it more consistent and effective. The shot is all about disguise so if you don’t have another shot that looks similar it will be seen to early and get avoided. Sometimes a soft little topspin can be just as effective as the FYM and way more subtle.
Ethics
This shot is obviously not intended to hurt anyone and normally we are all very well padded in the cold weather. At the same token, like everything in life, if you don’t commit too it its probably not going to work wonders for you. If you choose to hit this shot you need to be okay about getting one back. I think being sportsmanlike is very important no matter how intense it gets. So every time you use it make sure you apologize with a wave of the hand or racquet. Some might say you don’t mean it, but its the sportsmanlike thing to do.
Lastly, I would not suggest using this in mixed doubles or in your Sunday afternoon friendly if you want to keep having that Sunday afternoon Friendly!