Running router-modem as "multimode"?

(Originally posted this in Software, but it probably belongs here instead)

By way of background, I'm on a DSL PPPoE connection. Yesterday I couldn't connect at all, kept getting a 678 error (remote didn't respond). Finally called my ISP, and after running me through the usual checks, they finally tried walking me through converting my router modem from bridge to "multimode", whatever the heck that is, and that worked like a charm.

The guy explained that I'd probably experience more reliable connections this way, and the only disadvantage I might eventually face is that when and if DSL-2 becomes a widespread reality I won't be able to use it since multimode forces the use of DSL-1. The origin of the problem is still a mystery, but the guy thinks that for whatever reason, my bridge setting was trying to find DSL-2 and giving up when it couldn't find it.

Can anyone steer me to some reasonably non-tech info on this? I tried wikipedia, but wound up with hi-tech articles and discussions obviously aimed at IT's, which I'm not. Wrong search terms, probably, as usual.