Note: This page describes a glitch, trick, or exploit. To see others, click here.
Normally, to enter Kamui for the first time, you must defeat Ninetails to make thunderclouds appear in Shinshu, and use Thunderstorm to unlock the tunnel entrance. However, the loading zone to Kamui always exists under the dome, and it can be reached by KT'ing into it from below. This requires using KTs to get out-of-bounds at the Eastern edge of Shinshu.
In NG+ Any%, this trick can be performed early in the run to obtain Holy Eagle (Double Jump) early from the Kamui dojo. It is also a requirement in any route which performs Exit Oni Island to skip Ninetails.
"Kamui Early" is first mentioned in the Speed Demos Archive forum thread by Groobo on 19 August 2007 in this forum post, but from context it was likely in reference to KTing up to Shinshu Plateau early.
It was mentioned more clearly by eternaljwh on 14 August 2012 in this post as a hypothetical trick, but dismissed as being useless without skipping Thunderstorm. Groobo responded confirming he knew 2 methods, both of which likely involved KTs.