text: FM 7-100.1________________________________________________________________________________
  4-71. Engineer units specializing in rapid obstacle construction and minelaying
  form mission-specific units known as ODs. These ODs normally deploy in
  conjunction with reserves to block enemy penetrations or to protect the flanks
  of counterattack forces. In the initial stages of the defense, engineer assets
  concentrate on creating obstacles in the disruption zone, in gaps in the combat
  formation, and to the flanks, and preparing lines for counterpenetration and
  counterattack and routes to such lines. The obstacle plan ensures that the
  effort is coordinated with fires and maneuver to produce the desired effects.
  In conjunction with other tasks, engineers support the IW plan through
  activities such as constructing false defensive positions and preparing false
  routes. More information on countermobility and survivability planning at the
  operational level can be found in Chapter 10.
  MAKE USE OF COMPLEX TERRAIN
  4-72. The OPFOR tries to make maximum use of complex terrain in all
  defensive operations. Complex terrain provides cover from fires, concealment
  from standoff RISTA assets, and intelligence and logistics support from the
  population of urban areas. It plays into the strength of OPFOR resolve to win
  through any means and through protracted conflict if necessary.
  MAKE THOROUGH LOGISTICS ARRANGEMENTS
  4-73. The overwhelming ability of extraregional intervention forces to attack
  exposed logistics elements makes it difficult to resupply forces. The OPFOR
  understands that there is as much chance of a defensive operation being
  brought to culmination by a lack of sufficient logistics support as there is by
  enemy action. Careful consideration is given to carried days of supply and
  pre-established caches to obviate the need for easily disrupted lines of
  communication (LOCs).
  MODIFY THE PLAN WHEN NECESSARY
  4-74. The OPFOR takes into account that, while it might consider itself to be
  in the preparation phase for one operation, it is continuously in the execution
  phase. Plans are never considered final. Plans are checked throughout the
  course of their development to ensure they are still valid in light of battlefield
  events.
  REHEARSE EVERYTHING POSSIBLE, IN PRIORITY
  4-75. At the operational level, rehearsals are usually confined to map or
  sand table exercises to ensure understanding by subordinate commanders.
  The commander establishes the priority for critical parts of the operation,
  and rehearses those operations with his subordinates. Typical actions to
  be rehearsed in a defensive operation include
  • Commitment of a reserve.
  • Initiation of a counterattack.
  • Execution of the fire support plan.