They separate the suspended oil from the air stream by going through a baffle where it drips down into the container. Much better then re burning the oil in the motor and also creating a mess in the intake.

2003 Dodge 3500 Diesel

1964 Plymouth Savoy White 2dr sdn. 512 stroked, Dominator, New Best 9.75 @ 138.50mph
2019 Challenger Scat Pack 1320 White Knuckle-K&N Cold air filter, Billit Oil catch can, drag radials stock 11.911 @ 114mph