The idea behind this is to enable a player under attack in one City to move one's warriors from that city to another City owned by that player instaneously. or to enable troops to be moved instaneously from one City to another City of that player that is under attack.
ONLY troops can be moved no resources can be sent using this function.
Again this attribute can only be enabled once a relatively high level of of the Underground Cave is achieved.
To move troops en masse would have to cost a fair bit of some resource maybe corn and of course the City they are being moved to will have to have enough corn for their upkeep.
Once a complement of troops is moved from one city to another City of that player using the underground cave then they cannot be moved back to where they came from for a given period of time.
One has to envisage that the underground caves connect one city to any other city the player owns.