Perhaps it's analogous to a policeman's radar gun to detect speed of cars, though on a different scale.
In this case, on that scale, however, it's as if they're throwing a soccer ball at a piece of plywood to measure distance to such a potentially-resonating surface, presumably to note movement over time, indicating resonance. The resonator inherently moves not only in the case of resonance due to other influences, but also from the forcing function of the soccer ball.
HF