Questions for Singly balance mixer from AN1136

I'd like to know why in order to design input matching, S11, the IF port should be open-circuit terminated @LO/RF (instead of short circuit it) as shown in attached file (taken from HP AN-1136? How about determining S22 afterwards ?
Is there any similar @DS schematic to describe this picture ? cheer

This would make conversion more efficint.
Many people make test about the LO and IF terminal such as short/open/match...,but the short is most efficient.