A tachometer is used to indicate engine RPM.
It is normally connected to the negative terminal of the ignition coil. The pulses from the ignition primary circuit are then used as an input to operate the tachometer.
At idle speed, the frequency of the pulse is steady. As the engine speed rises, the frequency of the pulse increases, as the tachometer indicates.
As a safety measure, the tachometric relay uses an input (from the negative side of the ignition coil) to ensure the relay operates only when engine speed is above a specified minimum, say, 350 RPM.
In this way it can operate as a safety device, ensuring for instance that the fuel pump operates only when the engine speed exceeds this figure.
Source: CDX Global
