Concise description: drop on one territory and the other one(s) gets troops too. If attacked, both regions lose same number of troops. When attacking, both regions lose same number of troops, and if successful, troops from both regions are advanced (3 on territory A and 3 on territory B are still 3 troops). Reinforcing works the same way.
Example: (imagine this as a whole map)
fig 1: B is attacked (and can attack) from/to A & C
fig 2: B is attacked (and can attack) from/to D,E,F,G,H,A,J,K
Both B's and A's are the same territory and share the same number of troops.
To clear things: Attacking from H to A on the right, means attacking A on the left also. Territory named A has exactly same troops on both sides and both are deducted when getting attacked, and the attacker gains both of them. Just like polymorphic, but with territories!
To clear things #2: Imagine a map with teleports! You kill one side of a teleport device, and you don't have to kill neutrals on the exit side of the device, because your troops are already there!
This is similar to conditional attacks but it's a level more.
