After entering geostationary transfer orbit, the main engine of the Mercury Transfer Module will fire to boost the apogee (point of furthest distance from the Earth) of the Mercury Composite Spacecraft. The chemical engine will be fired around perigee (point of closest approach to the Earth) - indicated by the red tracks in the orbit animation.
Having reached the orbit of the Moon, a lunar gravity assist will next set the composite spacecraft on its interplanetary trajectory toward Mercury. Shortly after, the MTM's solar array is fully deployed and the advanced solar electric propulsion system is activated.
In the animation, all four ion engines are seen firing, whereas in reality only one will be switched on due to the limited power that is available. The exact direction of thrust can be selected by the steerable nozzles of the ion engines.