SUUNTO M3 - Manual (Page 9)

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2.3. USING THE BUTTON LOCK
In order to prevent accidental off synchronization of the race timer, you can activate or
switch off the Suunto M3 button lock by pressing first the ENTER and then the
CHRONO button within 3 seconds.
Note that the key lock only disables the following actions: starting the race timer,
stopping the countdown or race time and synchronizing the countdown.