I didn't see any mention of this and my friends in game didn't know, so I'll post it here.
Any time you use Sekkanoki with a WS that triggers aftermath, it gives you the effect of the TP you have when you WS, not the TP consumed by the WS.
I figure this is only really relevant to SAM or DD/SAM using a mythic that needs to be triggered by a crappy WS instead of their strongest WS.
XI Wiki

