By definition, the critical path of a project has zero slack when it is drawn up. That would be fantastic if all projects proceeded as planned, but sometimes a manager gets tasks completed quicker than expected -- and, more often, unanticipated events make everything take longer. When the latter happens, it means you have to figure out how to change your plan to get things back on track.
Critical path is a term that has its origin in project management. It requires the manager to list the required tasks needed to complete a particular project, the time each will likely take and the dependencies between the activities. These tasks are usually laid out in a diagram, with the project manager tracing the path from the first task to the last and noting the earliest that each task along that path can start and the latest it can be completed without causing a delay. That’s the best outcome for how quickly your project can be completed. It’s called the critical path, because it has no slack; every task on the path is critical, and a delay in any task causes a project delay.
Slack, or float, refers to the amount of days or hours that various related tasks can be delayed without pushing the project back. The critical path, by definition, has no slack. But other tasks not on the critical path may have built-in float. For example, if it’s a Wednesday and you’re opening your restaurant on Friday, but your dairy products haven’t been delivered, it’s not a big deal if there's only one day between order and delivery; you have a day of slack built in. On the other hand, if your training program for servers takes four days and you’re scheduled to open in two, that means you can’t open on time -- unless you change the path.
Schedule compression, or pruning, can occur when a manager decides that the slack can be eliminated by cutting some of the critical tasks out or completing them in less time. Maybe you decide that you can cut the training to two days by racing through certain sections or leaving out points you consider obvious or eliminating topics you think your employees already understand. That gets your project back on schedule -- but perhaps at a cost if the rushed-through training leaves servers unprepared.
A critical path can also recover from unanticipated slack if tasks once scheduled to be performed in a series can instead be performed in parallel -- at the same time. That can reduce the critical path, but it usually increases the risks as well. You might have originally scheduled the training to take place after your restaurant was built, but instead you choose to conduct the events in parallel. That might work to save time, but it may cause both events to take longer than planned or consume more resources because of decreased production and efficiency. It might be harder to conduct training with the noise of construction and harder to do the construction while maneuvering around a class.
Another way of overcoming slack on the critical path is crashing, or throwing more resources at a task to get it completed faster. In the restaurant example, that might mean hiring additional staff and make the training more specialized, so everyone doesn’t need to be trained in everything. Or it may mean hiring more trainers to provide individual attention to get the staff up to speed faster. In any case, it get your path back on schedule but at a big cost to your budget.