This is a pretty simple rule. At any point in the game, a player may elect to damage their own infrastructure, bases or production facilities, as part of a “scorched earth withdrawal.”
Scorching the Earth occurs during the repair units phase, except in this instance, rather than repairing their units at a cost in IPCs, the player will instead be dismantling them (placing chips under their bases/facilities rather than removing chips.)
option 1: Players may damage their own facilities up to the maximum value (twice the operation threshold) at no cost.
option 2: Players roll 1d6 (in the case of bases) or 2d6 (in the case of production facilities) to determine how much dismantling and destruction they can achieve before the enemy arrives.
[Edit: option 3 (scrapped, see comments below): Players may recoup some IPCs from this dismantling process, up to, but not exceeding, the operational threshold of the base or facility. Example, a player may recoup up to 3 ipc for a base (not 6), or 10 ipcs for a Major Facility (not 20) etc. Basically, even if the enemy is allowed to bomb up to twice the value, for the purposes of dismantling a base or facility is only worth its operational value.]
These rules create a means whereby a player can preempt an enemy advance on their infrastructure, and damage this infrastructure beforehand, to prevent it from being immediately used against them the following round. However, and this is critical, the player may not completely destroy the bases or facilities. This prevents a situation where players can completely raise bases or facilities to the ground. Here you have the option to damage, but not to totally destroy it or remove it from play entirely. So there is still some risk, or at least a cost/benefit aspect to the initial purchasing decision.