Emptiness is the track on which the centered person moves.

In A Field Guide to Getting Lost, (p. 50) Rebecca Solnit quotes the Tibetan sage Je Tsongkhapa. She goes on to quote Stephen Batchelor in Buddhism without Beliefs, who points out that Tsongkhapa uses the Tibetan word shul to refer to a track:

…a mark that remains after that which made it has passed by–a footprint, for example. In other contexts, shul is used to describe the scarred hollow in the ground where a house once stood, the channel worn through rock where a river runs in flood, the indentation in the grass where an animal slept last night. All of these are shul: the impression of something that used to be there. A path is a shul because it is an impression in the ground left by the regular tread of feet, which has kept it clear of obstructions and maintained it for the use of others. As a shul, emptiness can be compared to the impression of something that used to be there. In this case, such an impression is formed by the indentations, hollows, marks, and scars left by the turbulence of selfish craving.

The impression of something that is no longer there is a powerful idea. It reminds me, yet again, of Antonio Machado's powerful poem Se hace camino al andar.

Wanderer, there is no road, the road is made by walking.