This looks like a standard three-wire circuit for driving two motors, but upon closer examination, we see that motor starter M1 is controlling a motor pump for lubricating oil. The main drive motor, controlled by motor starter M2 is controlled by a normally open pressure switch.
By pressing the start button, motor M1 will start to pump lube onto the bearings of the main motor. Once the pressure meets the minimum threshold and closes the pressure switch contacts, motor M2 will be allowed to start. This arrangement ensures the motor M2 will not run if there is insufficient lubrication pressure.
The circuit will continue to operate as a standard three-wire circuit providing low-voltage protection (LVP) until either the stop button is pressed, or an overload occurs. If an overload occurs in motor M1, then M2 will continue to run until the pressure switch drops open. If an overload occurs in M2, it will stop while motor M1 will continue to keep lubrication pressure up.
Reference
Basic Motor Control by Aaron Lee and Chad Flinn is used under a CC BY 4.0 Licence.