The lock up will happen when you go into gear, if you hit a neutral the rear wheel won't do anything, as there is nothing to stop it turning freely.
Example: (All figures are made up, I can't remember what revs=what speed in each gear for an SV)
30mph = 5,000 rpm in 2nd = 8,000 rpm in 1st
Bimbling along at 30mph in 2nd you decide to change down to first. The engine has to go from 5000 to 8000 rpm. The only thing that can change the speed of the engine (if you
don't blip the throttle) is the rear wheel turning. However if the force of the engine is greater than the grip available to the rear wheel, the engine will stop the wheel, rather than the wheel accelerating the engine.
Instead, if you blip the throttle during the down change, you increase the rpm that the engine is turning at, so the rear wheel doesn't have to accelerate the engine at all (if you do it perfectly) and away you go.
It's all about matching revs to the gears your going to be in after the change, but you don't need to be spot on a healthy blip will do just fine.
All of the above is as I understand it, and may be totally wrong. I've got a slipper clutch now, so it's not as bad, but I still blip (I like the sound
)
What happened in the above cases (I guess) is that going into neutral allowed the revs to drop away much more than normal, so the rear wheel had EVEN MORE to do to accelerate the engine once a gear was engaged again. Hence the lock up.