Let me boil this question down to the basic argument: With the car in high gear does the clutch spring shove the crank forward there by reducing the gap between the coils & magnets ( to what ever it was originally set at) caused by crank end play?
The crankshaft is pulled toward the rear, wearing the 3rd main bearing's thrust face on the front end of it. The mag gap gets larger as the thrust face wears.
In high, there is no external forces on the crank. When the low pedal is depressed the crank is pulled rearward as Mike stated.
So jacking up a rear wheel and putting the car in high for an easier start on mag has no effect on the mag gap at all.
I guess if you had a lot of end play, the crank may slide forward due to the tilt of the car from being jacked up.
The rear wheels were usually jacked up to let the engine spin over easier due to dragging clutches or oil viscosity.
It might help... if your hand cranking and want to use the rear wheels for a better "flywheel" effect. I'm not sure how much benefit you'd get though as the engine would be harder to crank so it may have a slower rpm depending on your strength.
I've posted this before and it's actually the main point of an ongoing "discussion" with a fellow T'er. What is the purpose of jacking a rear wheel,putting the car in high, hand cranking and expecting or getting an easier start on magneto? That's why I boiled it down to the end play question this time. I cannot believe the flywheel business and suspected some thing was closing the mag. gap on an engine with a lot of end play therefore increasing the voltage which would explain an easier start. Let's not get into possible this stuck or that's out of adjustment the old wive's tale is: Jack up one rear wheel, hand cranking on magneto with the car in high gear makes for an easier start.
I've used that trick many times. But never heard of leaving the T in high gear....always in neutral or at least the 'best' neutral a T can have...that is normally the issue, so much cold oil, worn or dragging parts....puts more resistance to cranking over.
By cranking (either hand or with a weak battery on electric starter) the momentum needed for turning over the crankshaft to start, is helped by having the one rear wheel jacked up to able to spin free. Otherwise the T will try to creep as you crank, and if you pull the lever all the way back to lock the parking/emergency brakes, there is no reduction of resistance, from cold hard oil in the clutch discs....so you need to let a rear wheel turn free for easier start. That jacked up free wheel does the trick!