Maybe Mac can come up with a nice complex technical answer for you, Technical – but I’ll give you the nice simple one even I can figure out: it’s built into the YaBB software, and is probably totally ignorant of what browser (or even OS) you’re using.
At least one other place I visit regularly (and which uses Info-Pop, not YaBB), if someone’s getting impatient for a reply they often post a simple “bump” reply if it looks like the topic’s sliding too far down to attract attention.
And thanks, guys, for the answer to my original question (whatever that was ;D )