Overflow bottle - Bypass opening

The bypass opening in the vent hose is presented by Rotax as one of three options for protecting that hose from freezing shut.  It is assumed that any portion of the hose that is outside of the cowl could freeze, so cutting a small notch in the hose just before it exits the cowl would allow for pressure relief in the event the hose end freezes shut.

If the vent hose plugged completely it’s possible that the overflow bottle could become over pressured and rupture, even during normal cooling system venting.  Instead of the notch, you could also just stop the hose short of exiting the cowl, where it’s typically warm and would not freeze.  The overflow bottle is (and must be) vented even without the vent hose installed. The addition of the vent hose is not required, but using it directs any steam (contaminated with antifreeze) out of the engine compartment.