I wasn't sure what one arm route meant, but I was suspecting it is another term for "router on a stick". I googled it, and lo behold, I was right! This is a term that means the interface on a router is routing multiple subnets. This can be done a couple of ways, either by creating secondary ip addresses on that interface, or by creating subinterfaces of that physical interface.
For example interface FastEthernet0/1 would be the real physical interface. A subinterface looks like this; interface FastEthernet0/1.1.
The advantage to this method is in cost savings. Extra blades or modules for routers are pretty expensive. The disadvantage is that it doesn't perform as well as a layer 3 switch.
it also doesn't have to be routing different subnets or have different ip addresses or sub interfaces to be a router on a stick either. Although not an efficient method, a router with a single interface (and IP) can be the default gateway of all (or some) of the hosts on that particular subnet, routing traffic to other destinations.