dont rock it back and forth, i dont know if it has worked for anyone in the past, but thats not really the safest bet......
it would be much better to simply cut the byssal threads as far away from the shell as possible. pulling on it and even rocking it back and forth can and most likely WILL rip the byssal threads and with that runs the chance that the byssal organ will be torn out as well, and if that happens the clam will most likely die, or at least there is an extremely good chance of that happening. it will be much better to cut the threads with a razor.