Ammeter needles will sometimes dance disconcertingly ... pulses off the distributor or the negative side of the ignition coil. Mechanically driven tachometers work like the mechanical speedometers ...

c) The induced current, and hence the deflection of the ammeter, could be made bigger by: Moving the magnet faster. Using a stronger magnet. Having more turns of wire in the coil.

Note that this magnetic field opposes the original change. For example, if a magnet is moved into a coil of wire, the induced magnetic field tends to repel the magnet back out of the coil.