Most people have misconceptions about stopping the tenure clock due to extraordinary circumstances (what does it involve, when can you do it, etc...).
I know I did - it's not something that people talk about often, and some people view it as a weakness or failure. Luckily, Inside Higher education recently published a pretty good article titled Ignorance About "Stop the Clock" Policies that touches on a lot of the major issues.
In a nutshell, stopping the clock merely means that a tenure track faculty that would have to go up for tenure five years after starting would be held to the same standards of research (and teaching), but would have six years to make the grade. In other words, if a school required five articles in :good" journal to be published in five years' time, the faculty member would then have six years to publish the five articles.
It was particularly timely for me because I've been thinking about stopping mine for the year as I deal with the Unknown Son's cancer treatment. For a while, I was trying to convince myself that I could handle it all (teaching research, and my family). But sometimes you just can't. So this week, I went to my Dean, and he had no problem stopping my clock for the year. I told him I'll still teach my classes, since that actually serves a good break from the medical stuff. And in fact (though I didn't tell him this), I'll likely get as much (if not more) research done as last year. But now there's a lot less stress, and I can take whatever time I need to spend on my son without having to worry about whether or not it'll affect my tenure.