Ron is partially correct. Shiho Nage (what we refer to as Tenkai Kotegaeshi) is allowed in competition but it must be executed such that the other person's balance is broken to the rear long the line of the spine. In other words, the version that takes the arm out to the side, twisting the shoulder at the end, resulting in a kotegaeshi type ukemi, is not allowed in shiai due to a high potential for injury to the shoulder. This version can be found here - http://www.youtube.com/watch?v=-9F04U92suk
(excellent Yoshinkan demo btw).
An example of the allowed version can be found here - http://homepage2.nifty.com/shodokan/en/kyogi10c.html
. It can be clearly seen that Uke's arm is guided along his head and down his back as against outwards to the side.
From my own experience it's quite effective and can be executed with a lot of power, even if one does not twist to the side, however for me it works best when dealing with a taller person, which does not happen that often in my case.
Our basic counter for this waza is quite effective so one must
have good kuzushi to get it working under resistance. If I find a video with it being executed in shiai I'll post it.