Yes, it is a metaphor that suggests they can easily get angry. The expression is generally used for motorcycles whose “minimo” engine level is the level at which the engine can run when it is idle.idle.
Idling refers to running a vehicle's engine when the vehicle is not in motion. This commonly occurs when drivers are stopped at a red light, waiting while parked outside a business or residence, or otherwise stationary with the engine running.
At idle speed, the engine generates enough power to run reasonably smoothly and operate its ancillaries (water pump, alternator, and, if equipped, other accessories such as power steering), but usually not enough to perform useful work, such as moving an automobile.
(Wikipedia)