G'day Ian,
Good introductory explanations of Keyline will certainly depend on who your audience is... and
should be tailored to their interests as best you can.
But in general, if you're talking with ranchers/farmers/land-holders of some kind leading in with Keyline as a low-cost investment with a high soil fertility return across the landscape is a decent go... Of course the soil fertility point can be expanded a bit by explaining the whole landscape
water harvesting aspect, the fact that you get the most water storage in soil possible as evenly as possible over the
land, along with creating improved conditions for an explosion of soil biology... all leading to increased soil fertility which further reduces input costs (less irrigating, or more efficient irrigation, less fertility inputs, more money saved) for agriculture. Rapid topsoil creation is usually interesting to such folks!
Also, 'drought-proofing' possibilities tend to prick up some ears... so you can get into the notion of additionally creating connected farm
pond storages, that can also be used to gravity irrigate large areas with little effort.
If further interested from there can go on to explain that these approaches are a portion of a larger 'whole farm planning' approach that Yeomans' developed, and that has been used with success...
Anyhow, there is no truly simple or 'right' way to give an 'elevator pitch' about Keyline... just do the best you can, focus on who you're talking to at that moment and what aspect might most interest them, and use your natural born charm!