@CharlieB I believe he has said he verified that second curtain sync is maintained after he removed the Canon flash.
I took that to mean its maintained in the menu system... wasn't sure if it was actually doing 2nd curtain.
Well aware of all complexities with flash synch... you simply cannot have the start of 2nd curtain travel coincide with the synch. Synch has to be slightly before 2nd curtain travel begins, and it needs to be enough before that, so that the flash has finished firing before the curtain has covered any of the film gate (sensor gate?).
Not talking HHS here, but good ol' X synch. Doing a bit of research.... curtain travel time, complete traverse at X synch speed of 1/250 sec, is about 3.5milliseconds, or... 3.5/1000ths of a second. Clearly, if a longish flash duration of 1/1000 sec is called for, we're gonna have a curtain that is about 1/3 of the way across the sensor when the flash stops firing - IF - the synch is at the beginning of 2nd curtain travel. Therefore, actual 2nd curtain synch is really "near 2nd curtain" synch, whereas, the synch must occur with at least as much time before 2nd curtain travel, as it takes for the flash itself to fire, and to allow for any systemic delays.
So... I'd say, someplace at least around 1/1500th of a second before actual 2nd curtain movement, you've got 2nd curtain synch, which insures that a long duration (not HSS) flash is fully extinguished prior to the shutter actually starting to close.
How Canon handles that... speculative, but... its not unreasonable to think they take it into account, and vary the actual synch in relation to the 2nd curtain movement, based on the predicted flash firing time (not HSS).
As we know, with conventional flash photography, the flash duration, not impulse power, determines the effective flash exposure. The flash is made to fire for shorter times as less "power" is needed.
As far as I know, there is no 2nd curtain synch for HSS, they're mutually implementable modes of operation.