downloadable videos, the mechanism

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I'm attempting to understand both ends of what is happening with
online videos being viewed in web browser.

There are many free videos, like tutorial type stuff on the net and as
I experiment with some of them  I see the file is actually downloaded
to my pc in most cases.

For example, with firefox I see a file appear in the Cache that is
much larger than similar files, and as I go along clicking video links
and seeing them displayed ... in this case with Quicktime, inside
firefox, these beefy files pile up.

I know they are related to the video since if I clear the cache by
just deleting everything, then click a video link I see a similar file
appear and grow as the video is downloaded.

OK, so far so good.  These files linger until the Cache hold setting
arrives I guess.  So I thought it was a simple mechanism.  Video
starts downloading, video starts playing, and completes downloading.

But then I happened to accidentally delete the Cache while a video was
still playing... and guess what... it didn't effect its playing at

The cache had zero files, yet the video continues playing...

Then I noticed something else that seemed out of sync with how I
thought it all worked. And this appears to hold true in numerous

If the video is more than about 25mb, the instant the downloading part
ends the file disappears.  I know the approximate size because the web
page gives that detail about the files.  

So anything smaller lingers in the cache, anything larger disappears
the very instant the download ends.

The hard part to me, is that the video being played inside the browser
continues playing anyway, and can be restarted as much as desired.
I guess, I don't understand where it is playing from.  Surely large
files like that are not being held entirely in memory?

It appears that since the file disappears totally once the download
ends then why is it downloaded at all?  If the video can play with or
without a disc file present then why are they downloaded, and why do
they disappear above a specific size. and linger on disk below that

I hoped someone here can shed some light on these things or steer me
toward a source of explanation.

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