The beam is split and re-emitted in multiple points. By controlling the optical length (refractive index, or just the length of the waveguide by using optical junctions) of the path that leads to each emitter, the phase can be adjusted.

In practice, this can be done with phase change materials (heat/cool materials to change their index), or micro ring resonators (to divert light from one wave guide to another).

The beam then self-interferes, and the resulting interference pattern (constructive/destructive depending on the direction) are used to modulate the beam orientation.

You are right that a single source is needed, though I imagine that you can also use a laser source and shine it at another "pumped" material to have it emit more coherent light.

I've been thinking about possible use-cases for this technology besides LIDAR,. Point to point laser communication could be an interesting application: satellite-to-satellite communication, or drone-to-drone in high-EMI settings (battlefield with jammers). This would make mounting laser designators on small drones a lot easier. Here you go, free startup ideas ;)