In Dueling with Osensei
, I have a chapter entitled "There is no such thing as Tenkan . . . without Irimi" There, I make the point that I've seen no writings whatsoever of Osensei referring to Tenkan, Instead, he referred to IRIMI and IRIMI-TENKAN. (What Rupert has been writing above). This is a jujutsu response (that's not bad - I just mean it as phenomenology) - where the opponent is powerfully/skilled/lucky enough to neutralize your irimi, resulting in a tsuki (opening). In other words, the initial irimi was still successful by unbalancing the attacker, and tenkan is the method in which one defeats them.
For example, one thrusts at the opponent's throat, and has they forcefully bring their weapon upwards to deflect, you cut under their arms.
But: if you think about this in a one-two sequence, it will still not work, at least not against an equally skilled opponent for the following reasons:
1. Your initial attack will have been too weak, so that the defender will take the initiative
2. Your initial attack will be forceful, but unbalanced, so that you will never be able to "reboot" in time for a "2nd" cut. Instead, you will end of in a clash of strength. (grappling for position)
Which leads, full circle to my addendum on the Aikido Journal site, about the need to forge one's body
as a sword. That being accomplished through tanren to develop the "aiki body" - (yes, here we are again), one has the ability to thrust with full power, without any over extension, and with one's musculature and nervous system so "aligned" that when the initial move is neutralized, one flows
into the next cut. In other words, irimi-tenkan is one movement, not two.
If, for the sake of an image, one thinks of irimi as a line, then irimi-tenkan is a loop.