OK, a lot of examples in that link, does not explain the bonding.
Anyway, side on bridging: Is when one N2 pi orbital forms a bond with one metal and then a second metal 90 degrees to the first metal forms another bond with the N2 because N2 has another i orbital perpendicular to the original one.
Dihapto is just one pi orbtal overlaps with a metal when bound side on. When the backdonation is very strong for a side on bound N2 the double bond is formally broken, causing the dihapto binding?