[AtlUmpSteve]

It's quite simple, actually. U1 chases all fly balls from F9 to the right field dead ball line, U3 chases all fly balls from F7 to the left field dead ball line, and U2 chases all fly balls from F7 to F9. When anyone chases, the others revert to standard 3 umpire mechanics, with rotation clockwise; if no one chases, you cover your base. U1 and U3 work the lines; U2 lines up in calling position at 2nd (same place U3 sets up in 3 man with R1 on 1st).

ASA had 4 umpire mechanics that were taught at Advanced National Umpire Schools, and used in National Championship Finals; they were just never published in the Umpire Manual. When the staff changed several years ago, the new staff stopped recognizing it, because it wasn't previously available to the general umpire community. Quite a few of us still have the paperwork.