The Vision bypass allows operating small loads on Z-Wave controlled 2-wire dimmers.
In the two-wire system the electronics of the dimmer or switch is powered by one wire coming from the mains supply plus one wire that comes from the load itself.
This means that even when the load is turned off a small current runs through the load in order to power the Z-Wave electronic. This small current can cause a small load (e.g. a LED bulb with only 9 W) to flicker even when powered off. This flickering will also impact the dimming.