Shift T-56 with ignition cut-out?

I mentioned to someone who knows alot more about transmissions than I, that I'd like to utilize an ignition cut-out for faster shifing.
He said the trans would put up with it for a very short while and muttered something about the synchros..
Question is: Why is clutchless shifting during the millisecond the ignition is cutoff at speed so much more damaging than using a clutch with the engine screaming at max rpms?
I guess there's a good reason, but off the top of my know-nothing-about-transmissions head it seems that it'd be okay.
Can anyone kind spell it out for me?

power shifting (keeping it floored) keeps the engine revved up while shifting...

his issue with clutchless shifting isn't the engine, it's that you'll destroy the trans...

it wears out synchros and gears... quickly.

That was about what he had to say,and he sells tons of T-56s so he should know.
I was just curious what the mechanical difference was/is between a trans being shifted with the clutch in i.e 'load removed from the trans' and shifted with a momentary ignition cutout removing the load from the trans.
In both cases it'd seems the shafts are still turning inside the trans, the load's removed, and everything's happening about the same in either case.
I'm not talking about just jamming from one gear to the next w/o the clutch aka power shifting; at least not in its' normal sense. The ignition's been cutoff and the trans has no load on it. The trans doesn't know if the load/torque's been removed from it by depressing the clutch, or by the engine being off.
Or so it'd seem.
If there's an easy mechanical explanation why shifting with the engine shut off (at speed) is harder on the trans than shifting with the clutch in I'd sure be interested :)
Thanks in advance!

the difference is that the drive train is still connected with the clutch not depressed.
that means that you're going to have the load of the engine (regardless of whether the ignition is firing) on the trans when you're trying to shift...
much more stressful.

consider how many revs the engine is spinning (even with an ignition cutout) when you're shifting without the clutch. that's alot of weight/force, and you're transferring it manually to another gear, via a synchro. the synchro takes the pain on that one...
do the same, but when you push the clutch in, you're removing the weight/force of the engine, making it easier to shift.

note that when the car is in gear, and the clutch is out, how hard is it to get the shifter out of gear?
much harder than when the clutch is in...