At 50-53 that slack movement i have seen a few times and i’m wondering how to do it?
If we are talking about the same part what he is doing is a basic String Rejection while in a 1.5 mount configuration but instead of the string going into the gap right away he lets the extra loop whip over then catches it.
If you dont know how to do the basic string rejection I am talking about this video helps you get an idea of what I mean and will help you…http://www.youtube.com/watch?v=tktZLlivJTU
The trick is during: :34 - :44
So the trick your referring to on your video is basically one of those, with 2 differences:1. He rejects from a 1.5 formation rather than a trapeze. 2. He has an extra loop on his freehand and instead of just catching it right away like Hiroyuki does he whips the slack over so there isnt that extra loop anymore then catches with a whip rather than catching straight off the rejection.
Trying to explain some tricks using words is ALOT harder than using images, so if you have any questions or if I can help explain something else better please let me know =).