Anywhere, everywhere
Well for me, it auto morphs once you enter the piston part of the maze, but you can safely stand in that room at the end. And it may not be faster, but it is an intersting novelty trick, kinda like Ice Beam Before Flaahgra and a Varialess Magmoor Run. They are fun but do not help speed in any way.
And, Dr. Trence, it is a hard trick, I've only been able to do it twice. An easier way to find out when you are going to pop out of the tunnel is after each Magmaul Boost, stop. If you are quickly moving back towards the entrance of the tunnel, back up and get ready. One more boost will push you out. Once you are out make sure to hold back when you land back in the tunnel. If you land at the same spot you started, you will just fall through and land in the secretized room. And don't worry, you haven't even gotten to the hardest part yet. :P
And, Dr. Trence, it is a hard trick, I've only been able to do it twice. An easier way to find out when you are going to pop out of the tunnel is after each Magmaul Boost, stop. If you are quickly moving back towards the entrance of the tunnel, back up and get ready. One more boost will push you out. Once you are out make sure to hold back when you land back in the tunnel. If you land at the same spot you started, you will just fall through and land in the secretized room. And don't worry, you haven't even gotten to the hardest part yet. :P