Yep Snooze pretty much summed it up. It means if you make what ends up being a bad call (fixing for ages at a high rate like you and I both did in 2008

) it doesn't affect your whole mortgage, because hopefully the way you've fixed/floated the rest turns out better.
Of course the flipside is that if you've made a really great call, you won't benefit over the whole mortgage either.
We'll be splitting ours when it comes off fixed in November.