unsnaggable FLV

if a popup/online viewer lets an FLV through long enough to let me see it, why can't grab++ (orbit) etc snag it? this happens on certain sites.

obviously the site owner doesn't WANT people to snag his video, but i thought it was a lost cause protecting them? i mean, my PC is downloading the FLV in some form, right? so how does the software derail the interception software?

in such cases, grab++ just doesn't see the corresponding URL. except on one site where it actually DOES snag the URL but it sits dead in the orbit queue!! this is a first for me.

plugging the URL back into the browser, i get "URL is no longer valid". so, yeah, it's some sort of ad hoc/session URL -- same way youtube does it -- i get that, but then how can it still be valid when framed by the online player?

i'd give the URL in question, but it's got cheesy content (soft porn). given that spoiler, can i indeed post it so some FLV expert can have a look? i really wanna know what's going on!