It is turned on and off by just clicking the light button. The indicator on your display which shows what assist mode you are in flashes when it is active. It removes the speed limit from the bike completely. My speedo seems to keep up just fine. I have not had any problems with it.
The main thing to remember with these is that it can still be difficult to maintain high speed depending on which bike you have. Mine is on a haibike hardseven mountain bike so I'm in a fairly upright riding position. The aerodynamics of that just make it more difficult. I would say I'm a pretty fit guy, and do most of my riding on non ebikes. On a flat road if I'm working pretty hard with the speed limit removed I can cruise along at 26 - 27 MPH. It is still considerable work though. On my commutes to and from work which is about 14 miles each way, I usually average about 23 MPH. That is with a mix of flat and some hills.
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