There are even less details about this than there are about Hell's Ascension, however according to Sahne it is caused whenever an Ascension occurs. It manifests as a swirling vortex of dark energy in the ground of Heaven in opposition to Hell's column of light/holy energy that reaches into the sky. For each Demon and Median that would leave Hell, an equal amount of Angels and Medians are pulled from Heaven and down to Medius.
Sahne has also mentioned that Demons were performing a ritual on Medius to cause more Ascensions to occur. This means that as long as they keep doing this, more Counterreactions would occur and thus mean more Angels to aid in their plans.
Kieri, Kazai, Sanjulo and an unnamed Median were all pulled through one of these portals by surprise to become stranded on Medius at the cemetery where the Median in the group had been buried. Darius Elexion and a squad of Warrior Angels intentionally went through a similar portal during the Great War to fight on Medius, the presence of a Median and the location of arrival is not mentioned. Kinako and Tirol were also likely have pulled through the same way given her feelings toward it.