The Coanda effect in F1 is when exhaust gas is guided to go through the rear diffuser. The Coanda effect is fluid dynamics property when a fluid tends to follow the curvature of a surface rather after being in contact with a curved surface. The purpose is to increase downforce.
Using the Coanda effect in F1, the exhaust gases follow the contour of the bodywork. The exhaust system utilizing this principle is called Coanda-style exhaust.