Phasing is an F2LL technique used during insertion of the final block in the ZZ Method, specifically for the ZZ-b variant. It reduces the number of LL cases to a subset of ZBLL called ZZLL, thus enabling completion of the LL with 'one look' and significantly less algorithms than ZBLL. However, remember that the extra inspection required to do phasing still requires a 'look' during insertion of the final F2L block, but because phasing is very lightweight it should be possible to recognise and execute it relatively quickly.
Once Phasing is complete the LL edges will be permuted so that opposite colours (eg Blue/Green or Orange/Red) are opposite each other. If two opposite coloured LL edges are phased, then the remaining two will also be opposite each other. Looking at the LL after phasing the edges will be in one of two states. Either they are SOLVED or there is PARITY, which means that adjacent edges are not correct with respect to each other. An easy way to distinguish between SOLVED and PARITY is to attempt aligning the edges by rotating the U-layer. If its only possible to align two then it is the PARITY case.
As with intuitive F2L, some inefficiencies arise with intuitive phasing. Often forming a block with the remaining corner edge pair results in several extra moves. Further, these cases requires two looks: one to create the last F2L block, and another to insert it correctly. ZBLL gets around similar edge control issues by offering an algorithm for each F2L case. This is know as ZBLS. Advanced phasing is ZZLL's equivalent. There are three phasing algorithms for 18 of the 20 possible F2L cases, and two phasing algorithms for the cases where corner and edge are inserted but misoriented. This results in 58 algorithms total. Granted, it is questionable how much can be gained from memorizing every case. Some will save upwards of five moves while other none at all. Nevertheless, it reduces the process from 2-look to 1-look. Using advanced phasing with ZZLL can be referred to as "ZZ-b+". See external link below for speed-optimized algorithms.