"like a tennis player waiting for the service of his opponent"

You don't see too many sports metaphors in the neuroscience literature. But this one is not only rare; it is also spot on.

From "Key role of coupling, delay, and noise in resting brain fluctuations" by Gustavo Deco, Viktor Jirsa, A. R. McIntosh, Olaf Sporns and Rolf Kötter, PNAS:

Metaphorically speaking, the [brain's] resting state is like a tennis player waiting for the service of his opponent. The player is not statically at rest, but rather actively moving making small jumps to the left and to the right, because in this way, when the fast ball is coming, he can rapidly react.