According to the manual, it moves the Start frequency towards the Stop frequency where ever you have them set. So it just sweeps the frequency. Doesn’t control any of the other controls. When you have it in the toe up/heal down position, it doesn’t affect the frequency, and the start frequency is where ever you have it set

I’d assume (though I can’t say for sure because I don’t have one and it doesn’t explicitly state in the manual) that the envelope still functions while the expression pedal is plugged in, so if you wanted to just use the expression to control it, you’d just set the sensitivity low so you don’t trigger the attack.

That’s my assumption. I don’t see why they would have designed it so that plugging in an expression pedal would turn off the envelope function.

On most pedals I’ve used with expression pedals, rocking the expression pedal all the way back makes it like there’s no expression pedal there, and simply plugging an expression pedal in never turns anything off.