I've only been on the 777 for 5 years or so, with that in mind I think a detailed explanation misses the point. It's how it works in practise that is important, and of course how your SOP's fit into that.
Where I work we keep the A/T on the whole time, and I'm going to assume it's the same where you are.
The thing flies like a big C172, go faster and it wants to pitch up...so trim it out, slow down and it wants to pitch down...again with the AP disconnected..trim it out.
There, that explains "speed trim" or more correctly "trim for your current speed"
In practise I find It all rather intuative and it's one of the reasons I love the Jet, the cockpit seating on the other hand......
