What is the "ping pong" effect mentioned here?

Solution 1:

When you complete a Shaper-influenced map that is adjacent to an Elder-influenced map, the map you completed will usually flip from Shaper to Elder influence instead of returning to its default status. The converse is also true -- an Elder-influenced map will usually flip to Shaper influence on completion if it's adjacent to another Shaper-influenced map. This is the "ping-pong" effect that the post is referring to.

Shaper- and Elder-influenced maps have added monster packs and can drop Elder/Shaper rare items. These items can be quite valuable, as they can have mods which are not available on other rare items.