Overrun Combat can be found on page 86 of Campaign Operations, with a a similar version in Alpha Strike on page 107 of the Alpha Strike: Commander's Edition book.
It allows a force to Move and Shoot a number of units before the opponent has a chance to Move based on the Margin of Success of the Initiative Roll.
A question came up regarding the TMM of units that the Overruning units plan to Attack before they have a chance to Move. The units have not yet moved, making them have no TMM if we are to only track their Movement of this Turn.
Total Warfare does not address this directly, however The Battlemech Manual does state that the TMM is "...based on the number of hexes traversed by the target this turn..." (pg 25).
Due to the potential imbalances that can come in to play, we have played it as if they retain their TMM from the previous Turn, however it would help to have this solidified, either way.
Do targets of Overruning Units have access to a TMM, or are they considered to not have Moved yet at all?