Since such positions could not have resulted from best play, they and all branches of the game tree stemming from them can be ignored.
The null-move heuristic is designed to guess cutoffs with less effort than would otherwise be required, whilst retaining a reasonable level of accuracy.
The null-move heuristic is based on the fact that most reasonable chess moves improve the position for the side that played them.
In employing the null-move heuristic, the computer program first forfeits the turn of the side whose turn it is to move, and then performs an alpha–beta search on the resulting position to a shallower depth than it would have searched the current position had it not used the null move heuristic.
There are a class of chess positions where employing the null-move heuristic can result in severe tactical blunders.