"on the fly"

It means adjustment by ear while listening, in this case while playing a record.

@mesch is correct but be aware

1) Changing VTA changes VTF so you are hearing two things at once, and if you are making a large change to VTA then VTF will have changed substantially which is presumably not something you want

2) Unless you have an incredibly steady hand and a non suspended turntable you risk damaging your cartridge if something slips as you make the adjustment

So in my experience VTA adjustments should be done carefully and incrementally and rarely "on the fly" (Kuzma and others offer remote control adjustment which obviates #2 but still problem #1 applies)

Azimuth may be the exception but few tonearms allow this to be adjusted on the fly (my Kairos is one that does but even so caveat #2 above rules out my trying to do this)

I can’t believe that is possible with your arm....though it certainly is worth doing with stopping the turntable ..... Evalute, correct, evaluate.....long and tiresome job, but can certainly improve the performance