I heard that expression quite a few times during casts, but what excatly is it and why is it called like this?
It's a tactic where a Mothership is used to phase out (vortex ability) the enemy army during an engagement. Following that, own archons are dropped into the vortex. When the skill ends, all units exit the portal in close proximity, and the auto-attack of the archons, coupled with their splash, wipes the enemy army. The tactic has long since been nerfed, by making units exiting the field immune to damage for 1.5 seconds.