Flash video player, I want it to start buffering from various points

So, every flash video playing component that I’ve used, does a standard progressive download. But I’ve seen some out there that when the user drags the bar to a later point in the video, it will start buffering from that point.

How is this done?

The ones I use, you can’t even drag the playhead past the parts of the video that haven’t been downloaded yet.

In other words, if I want to see the last ten seconds of a video, I would want someone to be able to drag the bar to that point and have the video start playing and buffering from there.

I’m a bit confused on this, and would appreciate some advice.