The AirMech movement rules are on pages 108-109 of Interstellar Operations, and refer to some of the advanced vehicle movement rules on pages 24-27 of TacOps.
On page 25 of TacOps, there's a "controlled" Sideslip maneuver for VTOLs and Hovers. IO makes no mention of this maneuver (while mentioning unintentional Sideslips), but it would seem a logical thing for AirMechs to be able to do, since they can hover on raw thrust. Was this an oversight, or a deliberate decision to disallow this maneuver for AirMechs?