.wh

.wh n [xx]

The "when" request. When position n is reached, execute macro xx; negative values are with respect to page bottom. If xx is not supplied, remove any trap(s) at that location. (A trap is the position on the page where a given macro is executed.) Two traps can be at the same location if one is moved over the other with .ch. They cannot be placed at the same location with .wh.