Different Types Of Starter Motors

'Inertia' starter motors
Until the mid 1970's most British vehicles used an ‘Inertia’ Starter Motor. This uses the sudden acceleration of the motor when energised, over the inertia of a gear to throw it into the flywheel ring gear. 

The problem was that the gear would hit the flywheel,  spinning at high speed and literally force it’s way into mesh, causing damage to both gears. When the engine fired it went faster than the starter motor and threw it back out of mesh. 

This crude system is still used on various small engines e.g. lawn mowers, outboards etc, but on the whole has been superseded by the pre-engaged starter.

 'Pre-engaged' starter motors
The pre-engaged starter does what it says. It engages the drive gear into the ring gear prior to the motor itself spinning up. On the whole this prevents wear to either of the gears.