Pixellate only part of a picture

TLF wrote on 4/12/2006, 1:03 PM
In documentaries, when there is a need to disguise a peron's face (or an offensive slogan, etc) the offending part is often pixellated or blurred, or disguised in some such way.

How can I achieve this in VMS? I'm guessing some kind of mask is required, but I can't figure out how to do it. No doubt is is so simple... when you know how!

Many thanks,

Worley