I first saw this example on Mythbusters. Its the myth of the instantaneously freezing beer. Basically what happens is that someone takes a really cold beer taps the bottle down on a surface and it freezes within seconds. The technical explanation from what I remember is that the liquid is at or below 0 degrees but its the agitation of the molecules by the tapping of the beer bottle on the surface that forms ice crystals which cascades throughout the bottle until it freezes. I think this process is a nice analogy for how digital can spark conversations within campaign and markets.
The above the line campaign creates the conditions for conversation [just like the freezer cools the beer] but it is the spark, the catalyst of a good digitally driven idea that allows for conversation to actually happen. The reason I think this is the case is that in order for people to talk about something they not only need context but they also need a handle of conversation, a unit of information or an action that forms the immediate reason for conversation in that social or social media context. It could be the sharing of video in Facebook, it could be the downloading and use of an App, it could be the allowing and use of a Facebook App, or it just could be interacting with something in a new way.
I think though without the catalyst, the spark, conversations that could have been virulent may indeed never get to happen or dont happen as quickly, driving the conversation to a critical mass and earning the brand fame. So just like the beer a campaign idea needs a tap, a jolt, a spark that catalyses conversation potentially taking the whole idea to another level.