The force(source) that makes ETT to just start(begin) at high speed. Mechanism of the impulse like for eg. Spring or magnet etc.
ET3 capsules are accelerated with an linear electric motor (LEM). The LEM uses electrical energy to create a magnetic field in attraction and/or repulsion to propel the capsule. One up to speed, the capsule coasts with only a small amount of power to keep proper timing and spacing.