Rakhi was the system by which the misls took a fifth of a village's produce in exchange for protecting it, from other misls and from anyone else. It was collected village by village and it was the fiscal basis of the whole misl period.
It was not a tax in the sense of a state's tax. There was no assessment, no register and no appeal. It was an arrangement between a sardar and a headman, renewable, and it moved when the balance of force moved.
British and Persian writers treated it as extortion. The villages themselves appear to have preferred it to the alternative, which through the 1750s and 1760s was Afghan columns living off the country. The evidence for that preference is that the misls could raise rakhi across an area they had no garrison in.
Ranjit Singh Ji replaced it with a conventional land revenue administration, which brought in more and was resented more.
Rakhi is the mechanism by which a stateless confederacy taxed a settled agricultural society for fifty years, and there is very little written about it.
Rakhi was protection sold: a village paid a fifth of its revenue to a misl in return for defence against everybody else, including other misls.
It was not extortion in the ordinary sense, because it was cheaper than what the collapsing Mughal administration charged and it was actually delivered.
It is how the misls converted armed strength into territory. A village under rakhi long enough became a village under that sardar, and the map of the twelve misls is a map of who was collecting where.