Also known as Bi-value Universal Death this is mostly an end-game strategy that relies on the puzzle having a unique solution.  Essentially if you arrive at a board where all the remaining unsolved cells have two candidates (bi-values) then you’ve done something wrong.  There are either no solutions or more than one solution.  That’s the bi-value universal grave.

There are a lot of implications for BUG, but the easy rule is as follows:

If your Sudoku has ONE cell with THREE candidates and all other undecided cells have TWO candidates - you can immediately fill that three-candidate cell. Just check which candidate appears THREE TIMES in the row, column or box in which this three-candidate cell resides. That candidate is the one that goes in the three candidate cell.