What is the working principle of gravel kits? In this answer gravel kits are mentioned. I then looked at the wikipedia page and I noticed that they are quite different from what I was expecting:



I was expecting some kind of "shield", plate or extended inlet on the bottom side of the engine.
Wikipedia says that these instead are


  vortex dissipators, using compressor bleed air


How do they work? How does a beam and a sidewise jet of air prevent FOD ingestion?
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.