The horsey can make any one of the eight possible knight's chess moves. On each move he leaves behind a footprint.
Horseys reaching the edge of the enclosure are deemed to have escaped. Some find their way out faster than others.
Using an exposure method* brightens areas where the horsey treads on already-visited terrain: the greater the number of visits to one particular position, the more finely divided the exposure ramp (i.e. if the maximum number of visits to a position is 3, there will be 3 different levels of brightness represented in the final image).
Introducing multiple horseys leads us on to Horseys II.