The slots are the air source for ventilation. At speeds above 30 MPH, the airflow past the end of the road draft tube creates enough vacuum to pull air out of the crankcase. The air coming into the valve cover supplies the flow that ends up coming out of the draft tube. Use a vented oil fill cap to get the same result from the non-slotted cover.
Jerry

"It is better to be silent and be thought a fool than to speak and eliminate all doubt!" - Abraham Lincoln
Cringe and wail in fear, Eloi- - - - -we Morlocks are on the hunt!
There is nothing noble in being superior to your fellow man; true nobility is being superior to your former self. - Ernest Hemingway
Love your enemies and drive 'em nuts!