Refill keys are intended for use by engineers or owners/operators of machines to perform several machine administration tasks. These include:
However, the majority of refill keys are sold online due to the high demand for an advantage over other players. Both original refill keys and cheap copies can be found on a number of auction websites (and dedicated refill key sellers).
Refill keys give the user an advantage over the normal players of slot machines through several means. The main advantage is knowing the amount of money in the machine and the last bank and last win amounts - this information can prevent you from pouring money into a machine that isn't going to pay out soon. Having the knowledge that a machine recently paid out a substantial amount will allow you to choose a different machine.
Along with use in slot machines, refill keys can perform tasks in other machines; the most important being pool tables. Using the same size key slot as before, refill keys allow the user to release all the balls in most UK pool tables, giving you a free game.
Refill keys generally are priced at anywhere between £2-10 from online sellers. Even higher prices are available from scammers who make false claims about the ability of refill keys to save (or even earn) you money.
In addition to the key, some sellers may include a user manual and increase the price accordingly. These manuals are usually copied from sources on the internet, and rarely contain anything which could not be found through some quick research.
Refill keys are totally legal in the UK, but as you will hear from anyone who has used one - don't let people around you see what you are doing. Many arcade operators are unaware of the legality of refill keys, and after seeing you inserting a key into their machine will become quite angry. Even those who are aware of the legal status of their usage will still be unhappy about you gaining an advantage over other players.