How to tear down a geom mirror?

[Peter Steele]

>Yes. The "clear" commands usually just zero-out the last sector of the 
>underlying provider (doesn't matter if it's a drive, slice or something 
>altogether different) so you don't have to do it manually. 

So, as a generic solution then I could just iterate through all slices of all drives and run "gmirror clear" on each, and run dd to clear the first sectors. What btw is in these first sectors? I use this command because I saw it being done in one of the gmirror tutorials. I understand what the gmirror clear command does, but what is the dd command clearing?