Answer:
If you adjust your project schedule so that two or more tasks that were to take place sequentially will now take place in parallel, thereby reducing the timeline, this is called "Fast Tracking".
Fast tracking is the schedule compression technique in which phases or activities normally performed in sequence are performed in parallel. Production plants have this technique followed in case the process is steamlined/sequential like assembly line. Even service organisation have been following this technique in many ways.
Example:
Steps 3 to 11 can be go in parallel
Also, one may have 3 cutting machines, 12 sewing machines, 7 labor in each stage of stitching the shirt, 6 inspector....
The Case of Shirt Manufacturing:
Step1 - Cut
Step 2 - Attach S
Step 3 - Sew
Step 4 - Hem
Step 5 - Attach Co
Step 6 - Collar
Step 7 - Sleeve
Step 8 - Front
Step 9 - Back
Step 10 - Shoulder
Step 11 - Cuff
Step 12 - Iron
Step 13 - Inspect
Step 14 - Fold & Pack
Crashing is a schedule compression technique that is used to reduce/shorten the duration of a project without amending the defined Scope. It largely depends on the Project Sponsor's agreement. Agile ways make the Project Manager be more flexible and deliver ahead of schedule date or fixed the delivery date when the project was delayed and not meeting the schedules but you made sure it is delivered as scheduled to make up the lost time. It can be stressful for resources. Adding resources or expert/experienced users makes a big difference to outcome.
Both the techniques, fast tracking and crashing, considering the cost and schedule trade-offs, are utilised in order to determine or compare best output/results.