Ma Lin was even better at this type of shot - the look of bewilderment on the opponent's faces was priceless.
IMHO, Ma Long tends to make this shot when two conditions are true:
* the opponent makes a slow spinny, relatively shallow, topspin shot into ML's BH
* ML himself is out of position - because the topspin is not deep - to attack it properly
I agree with you vvk1.
The concept of the stroke follows the same logic behind looping a long/shallow push. The push comes with backspin, we have to brush it faster than the rotation on the ball to put it on the other side of the table. So the ball comes to you with backspin, and comes back to the opponent with topspin.
When chop-blocking the ball comes with topspin, and goes back with backspin. So in order to do this move consistently, its easier to do it with slow and shallow topspins, due to the short stroke of a chop-block (you would have to do it extremely fast with fast and heavy topspins). Check this video of the Kenta's block which follows the same structure.
Ma Long's block follows the same essence, but players tend to chop-block always more perpendicular to the spinning axis, since the rotation strength decays when closer to the poles of the balls.
This mini-article from Table tennis masters pictures what I'm trying to say:
http://www.tabletennismaster.com/profiles/blogs/service-receive-secrets-from-japan
The article shows about the chiquita backhand receive, but the concept can be transfered to the chop-block.