Fast tracking is a technique that is often implemented in crisis and/or crunch times so to speak as it involves in taking a specific schedule activity and/or work breakdown event that has been previously scheduled and/or is underway and expediting it in some way or another. Fast tracking is referred to as a project schedule compression technique of sorts in that its intent is to take an entire schedule of a project and attempting to compress it into a smaller period of time by conducting some events either quicker or by doing some events that were intended to be done in a more spaced out manner but rather doing some of them simultaneously. The network logic has essentially been changed allowing for some items that would otherwise have been done in a sequence are instead overlapped as such. For more information please see the more general umbrella term of schedule compression and or crashing.
This term is defined in the 3rd and the 4th edition of the PMBOK.