The major selling point of the IR18 so far has been that it “moves” most of the downforce creation from the top side of the car (through the use of wings and wickers) to the bottom side of the car (through the use of ground effects). Ground effects have the advantage of being less reliant upon how calm the air surrounding a car is, which means the IR18 can get much closer to the car in front without experiencing the dreaded aero-push.
In the fledgling days of aerodynamics in car racing, much of the knowledge came from the aero-space industry. Basically, cars of the day began to be fitted with upside down airplane wings in order to generate a downward force, rather than the lift you see in an airplane. Over time, these wings were refined to reduce drag and produce massive amounts of grip in the cars, compared to the wingless counterparts of a mere decade before. That all changed with the Lotus 79; the first car to successfully employ ground effects for a full season in the 1978 F1 Championship. A season it dominated. (It is also the car used by the sister Lionheart Retro Series presented by HPP Simulation.)
The Lotus 79, and all ground effect cars since then, have relied on a different principle to generate downforce from the bottom of the car; the Bernoulli Principle. Essentially, the Bernoulli Principle states that an increase in the speed of a fluid occurs simultaneously with a decrease in pressure.
The underside of the IR18 is shaped such that the bodywork on the bottom of the car rises the further back you go. This means the air that enters in from the front in a limited area quickly rushes in to fill all the space it suddenly finds before it. This causes the air to accelerate, and as Bernoulli predicted, creates a low-pressure area under the car. In plain English: fast moving air can create low pressure, aka a vacuum. This is how the IR 18 turns a small amount of air into a large amount of downforce and allows us to enjoy closer IndyCar then you have ever seen before.