fair/foul tag up mechanic

[TBBlue]

The Toronto Blue Jays practice facility has backstops 15-20 feet from HP. Makes for lots of exciting plays at the plate on passed balls, especially at the higher levels with athletic catchers, pitchers and runners.
We work the RF line with PU going out and making the fair/foul catch/no catch plus the tag at third, in the manner stated by the above posters. In the pregame we go over with the BU to check all three bases, (if necessary), as backup when PU is on the RF line. But the tag at third is PU call, unless he specifically defers to BU when the appeal is imminent, which is rarely done.