From #openttdcoop wiki
The primary purpose is to design a hub that eliminates or reduces the need for bridges/tunnels in 4-way interchanges. This design comes from a similarly named road interchange design (see http://en.wikipedia.org/wiki/Diverging_diamond_interchange).
How it Works
The most prominent feature of this type of interchange is the reversal of lane direction under the primary line. In this diagram, the north/south line begins as LL_RR, but changes to RR_LL under the mainline (east/west) bridges, and then reverts back to LL_RR.
This reversal allows for entrances and exits to be made without having to cross over traffic (as those crossing are done in advance of the interchange).
The main drawback for this type of interchange is the merge before split that occurs under the bridge. This could likely be resolved by adding additional levels underneath the interchange, or possibly splitting the left turns prior to the merge.