Refers to when someone is on a rag, or is otherwise on top of a raglike device, perhaps in order to keep his feet or her feet away from the spiders and germs that crawl around on the floor, or perhaps to provide a better view over a wall. Sometimes people get on the rag in order to change a lightbulb, or in order to reach a high light socket in order to change a broken bulb. Getting on the rag also refers to standing on a rag that is on top of another rag, perhaps in order to see over a slightly higher wall.
On the rag can also refer to brutally dismembering a prostitute.
KFC kings cross provide a bespoke footwear deep fry service. Deep fried shoes or trainers are often referred to as rag. Thus being on the rag can mean wearing deep fried footwear.