PMP Exam Tip: Crashing & Fast Tracking explained...
Crashing and fast tracking are techniques used in schedule compression. Schedule compression is used when the desired result is to create output faster than indicated in the original project, without having to change the project scope. This is particularly beneficial if a competitor is working on the same type of project and you need to finish first. It may also be a company decision to complete a project more quickly than originally scheduled based on various factors.
The two techniques used for this process are referred to as crashing or fast tracking.
Crashing: When the crashing approach is used, any additional costs associated with rushing the project are reviewed against the possible benefits of completing the project on a faster timeline. Additional items to consider when using the crashing approach include adding more resources for the project, allowing additional overtime, paying extra to receive delivery of critical components more quickly, etc. Crashing only works, when adding more resources will lead to a faster completion of a project. For instance crashing will not work by adding more resources to "the concrete in the foundation has to dry for 3 days".
Fast Tracking: Fast tracking is applied by re-scheduling various activities within the project to be worked on simultaneously instead of waiting for each piece to be completed separately. This method is best used when activities can be overlapped. The risk involved is that problems can occur if parallel aspects of the project include dependencies. So if you work on design and production at the same time your risk is that you need to rework production if the design is change half way through the process.