From operators manual:
Controls
Operator Presence Control (OPC)
Levers
Located on the upper handle assembly directly above
the handle grips.
When these levers are depressed, the OPC system
senses that the operator is in the normal operator's
position. When the levers are released, the OPC
system senses that the operator has moved from the
normal operating position and will kill the engine if
either the transmission shift lever is not in the neutral
position or the PTO is engaged