It states it on page 25 of my manual:
How to ride
Before you turn the bike on and start riding, you should
understand the way the pedal torque sensor works. Mounted
in the drive-side rear dropout, the torque sensor measures the
very slight deflection in the dropout plate that occurs when
you pedal. Based on this measurement, the sensor outputs a
signal to the controller which is processed further and ultimately
translated into motor output.
In order to properly measure relative deflection of the sensor,
the controller first needs to recognize a “zero point”. This zero
point is calculated each and every time the bike is turned on,
immediately after you push the power button.
Because the sensor zero point is the basis for all measurement in
the system, it’s important that it be very accurate, considering
the conditions the bike is being used under; most importantly,
the weight of the rider.
Because of this, we recommend you follow this procedure every
time you turn the bike on:
1. Sit on the bike, putting as much of your weight on
the saddle as possible. Put your feet on the floor
for balance, but try not to rest too much weight on
2. Press the SET button on the display to turn the bike
3. Wait a second or two, then begin riding.
For the best ride, do not put your feet on the pedals, and do
not begin moving the bike until after powering up the system as
recommended above. Although the software is fairly forgiving,
it’s possible that the bike could be too sensitive, or not sensitive
enough to ride ideally if the procedure isn’t followed.