'Crash the project' is a term is used frequently in PMP materials. What does it mean exactly?
How is it related to 'compressing the schedule', if at all?
Crashing is simply the concept of throwing more resources--be it money, tools and machinery, humans, etc--at a work package in an attempt to decrease its overall duration. The general idea is, if you planned 10 days with one person to do a task, then applying a second person will decrease the duration to five days.
The issue is, this does not work that cleanly in real life. There are a ton of environmental and random variables at play that affect performance, as well as the resource elasticity of the task, where crashing may have no effect at all or actually make things worse.
For example, moving boxes from point A to B would have a ton of resource elasticity. Throwing more box movers at the task should certainly decrease the overall duration. However, the site and situation of both point A and point B may decrease the efficacy of crashing as you may clog the path between point A and B with more box movers and then slow things up.
A task with zero resource elasticity is like driving from Point A to B. Throwing another driving in the car will do nothing to get to B faster.
These concepts can easily be applied to more complex projects such as construction and IT.
EDIT: Gestation is a common example of how crashing does not work, but it is also not an accurate example. While we always consider pregnancy as nine months, gestation is probabilistic ranging from as few as 22 or 23 weeks to 46 weeks. So one could crash a pregnancy duration by introducing a prostaglandin to induce labor. A resource is not just human but is any and all resources, including this medicine. Therefore, a pregnancy does have some resource elasticity albeit with a high degree of costs and risks.
There are 2 strategies for compressing project schedules; crashing and fast tracking.
Crashing refers to adding more resources to critical activities in order to reduce the duration and save time while fast tracking is an attempt to make activities that would have occurred in sequence (one after the other) to occur in parallel (happen at the same time with little or no delay between them).
Although these strategies can be applied to projects, there are several things that should be considered. One key factor is that not all activities can be shortened using these measures and other strategies or a combination of strategies should be put in place to address such situations.