LifeSpan SP-870 M2 Owner's Manual page 11

B.) Emergency Brake
The same knob that allows you to adjust the
tension of the bike also doubles as the
Emergency Brake.
Use this safety feature in any situation where
you would need to get off the bike and/or stop
the bike's flywheel.
To use the Emergency Brake function in any
situation, firmly press down on the Emergency
Brake & Tension Control Knob (#30).
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SP-880