This is achieved in a variety of ways, but essentially involves creating a raised groove or area along the posterior border, on the fitting surface. This is often referred to as the postdam.

Raised-face seat. A groove around the seat collects particles pushed aside by the knife and prevents clogging. When the valve opens, media pressure cleans the groove.

The pedestal is arranged with a raised column and a groove, and can be connected with a first drive mechanism, therefore, is controlled to move in the horizontal direction.