It's directly under the 'enable auto assist' button, right?
If that's the one, it controls how much the assist reacts to your heart rate. If you turn off 'auto assist' does it disappear?
The assist cuts out at 15.5mph (roughly - depending on your wheel/tyre diameter), and of course when you stop pedalling as it's pedal-assist, not an electric motorbike. It will pretty much match your power output watt-for-watt until you reach 15.5mph, so you might not feel any assist at all when rolling on the flat. It really kicks in when going up hills, as it will give you up to 250w of power provided you're doing the same.

If you're pedalling lightly, then the assist will also be light.
The controls for how the bike reacts in each of the 3 power levels come under 'engine maps' which only appears when you have your phone paired with a bike and the bike switched on.