I don't know, for me, I have never really felt the need to specifically train for this, its really just a mindset that I have always just instinctively had. If anything for me, its actually almost easier. Because the only time you have a chance to do it is if you are far to the right or left, and when you are there, its only natural to go around the net. You just can't allow your mind to be restricted by what the normal perception of the table is, and put a false importance on the net. Once you are able to do that, and just think about getting the ball to the other side of the table, hitting it around the net is actually kind of natural, and if anything, its actually almost easier, because you don't even have to worry about whether your spin is correct to make it so the ball is high enough to get over the net, in deed the lower it is, the more effective.