Ok now that I've seen it, I have a couple of comments.
1) It's not the metal being up against the rostrum that is doing it. The same reaction can be obtained using bare hands. So it rules that out instantly.
2) It's not the "immitating male sharks" because the trick works on both sexes. If it worked on male sharks - then there would be *** sharks. And we all know *** sharks are really just dolphins.
3) Also the shark still has control of itself. It's not going into some state of relaxation and unawareness. They move at any point they want - even if you are still in contact wtih the rostrum. (Good way to donate a hand to science)