Complex sentence: Pressurization requires that all joins in the waveguide be airtight, something which is usually achieved by means of a rubber O-ring recessed into a groove in the face of at least one of flanges forming each join.
Split into simpler sentences: Pressurization requires that all joins in the waveguide be airtight. This is usually achieved by means of a rubber O-ring recessed into a groove in the face of at least one of flanges forming each join.