Fridrich (CFOP) Method
In the normal cases, it is assumed that the other 3 slots are filled, so the algorithms provided do not disturb them. However, unless you are on the last F2L pair, this is not the case. Using "free slots" (slots that have not been solved yet) can help reduce the number of moves needed. This technique mainly works with the Case 2 and Case 3 cases, that is, the ones in which both pieces are in the top layer.
In the Case 2 cases, notice how the series of moves pairs up the pieces. First the corner is moved into the bottom layer, then the edge is moved around so that when the corner is brought back up, they either form a connected pair or a separated pair.
In the Case 3 cases (except for the special case one - ignore that one), notice how the series of moves pairs up the pieces. First the edge is moved into the middle layer while the corner is reoriented so white is not at the top, then the corner is moved areound so that when the edge is brought back up, they either form a connected pair or a separated pair.
When the corner (for Case 2) or edge (for Case 3) is moved into the bottom layer, a slot is brought into the top layer. Which slot is it? It's the one that you are currently solving for! Why? Because that one is for sure empty! If you moved an already solved slot up while pairing the ones you are working on, you just destroyed the other one!
The idea of using free slots is that, if another slot is empty, you can use that slot instead to do your corner- or edge-moving-around.
Examples coming soon.