The standard line of play in jiu-jitsu is to attack the underhook, and most players do this for a number of reasons. I was trained always to attack the underhook when escaping to the knees, though it is much less aggressive an escape than attacking the overhook, so it will not get the same dramatic sweeps that an overhook can often score. Anyway, here are the reasons to prefer the underhook line, especially against wrestlers:
1.) It protects your base better. Most wrestlers are going to try to put you on your back again as soon as you attempt to escape to your knees. Having the underhook means that the inside arm is available to protect your legs, while having the overhook means you have to really work to secure it so that they can't use that inside arm to grab a leg and toss you right back to your back.
2.) It allows you to attack sweeps very aggressively. I find that I really like attacking the legs as I'm escaping to my knees, and having the underhook means that I can transition into all manner of shots or opt to work the move like a Greco clinch. Instead of just spinning out and smashing my opponent to the mat, which is my primary line if I end up with the whizzer grip from that position, I prefer to attack either the inside leg or even, if my opponent gets really loose and doesn't go for a whizzer themselves, even attack the back.
3.) It opens up the backdoor. This is my most important reason, especially as a smaller guy. I find that a lot of opponents have to take a second to set up the whizzer once you hit your knees, which can give you a second to really close the space between your armpit and their body, and put you in a position where you're attacking their back fairly aggressively. I rarely ever end up in a position worse than a low side control if I establish that close position.
If you do decide that you prefer the whizzer position in spite of this reasoning, my suggestion is to attempt to use your outside arm to attack their outside arm or their outside leg, as that puts you in a very good position to sweep them, either putting you in their guard or ending up in side control. Remember, though, that if they have the underhook, you have to keep that whizzer very tight to protect your back.
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