This is a rather crude, but effective, device to break-up compacted soil such as clay and hard-packed soil. It can get a bit messy but the combination of water and high-pressure air really did a great job with my lawn.
The concept is fairly simple, introduce water to soften the soil. Turn the water off and inject a short burst of 140 psi air into the ground. It will take several attempts but what it does is creates voids and fissures that water can pass through. Each blast of air creates more pockets and more channels for the water to flow. Eventually, at least in my case, you will see earthworms shooting out of the ground and spurts of water all around you. I experienced the ground swelling all around me.
The end result is a lawn with deeper grass roots. This is a very slow process and would not be very practical for most homeowners unless you like playing in the mud.