Everytime the player pings, it traces some rays to get the distance to all of the different colliders around. It uses this to build a mesh like this 
, with collisions coloured white the closer to the surface, and grey if there was no surface in range. Then it's just a shader which only colours in pixels if they are either in a expanding ring from the center, or if the ring has passed them and the pixel was white.
The better way of doing this would just be to use a particle system that has no gravity, is emitted all at once, and collides with the terrian, but Godot doesnt have particle collisions yet, and i couldnt be bothered learning a new engine