At the time of this writing, DARKTHOUGHT relies on the selective deepening powers of five different search-extension heuristics.
In order to avoid possibly explosive growth of the search tree as caused by excessive deep-search extensions in the case of repeated mutual mating threats, DARKTHOUGHT restricts them to null moves at depth = 2 in the first 2 * iteration number plies on all paths. For recapture extensions DARKTHOUGHT employs a HITECH-style definition based on material balance instead of full evaluation scores: recaptures are captures or Pawn promotions that lead to a similar material balance as that of the root node. Pawn extensions relate to moves of advanced Pawns to the 7th / 2nd rank (for White / Black resp.) and all non-capturing moves of Pawns to the 6th / 3rd rank.
According to literally thousands of test runs these extensions combine extremely well with one another and the rest of the search, especially with futility and null-move pruning (see  for an independent confirmation). Still, DARKTHOUGHT attempts to reduce the number of meaningless extension triggers by restricting some of them (e.g. Pawn extensions) to the lower parts of the search tree up to a certain, possibly adaptive depth threshold. Such extension layering requires careful fine-tuning before it works without intolerable reduction of the overall extension benefits. Fortunately, this seems to be its only disadvantage.