What is the working principle of gravel kits?
Notice the path of condensation leading up from below this engine into it:



That image was taken from this video, and it clearly demonstrates that there exists a path of air which flows from ground level up into the engine. Any debris on the ground is liable to be sucked up in that vortex. The nozzle is designed to break up the flowing air on that specific, well-known path.

Note that even when there is no condensation to make the vortex visible and obvious, it still might exist.

Here is a nice intake vortex video, and here you can see the vortex swirling on the ground. This video really shows off a C-17's huge vortex during landing.