Underneath "What is in play betting?" sits a method that takes two bets. Matched betting turns a book's free bet offer into a small, mostly predictable sum. The bettor backs an outcome with the book and lays the same outcome on a betting exchange, so that whichever result comes in, one side covers the other. The profit comes from the offer itself, which is why the method depends entirely on the promotions books choose to run and on following each step with care.
Is matched betting legal? It uses ordinary bets and ordinary offers, so it is legal wherever betting is, and the offer terms are the only rules in play. Books do not welcome it, though, and many watch for it. An account that only ever takes offers and never bets otherwise is often restricted from future promotions, and that restriction is what ends most matched betting runs well before the offers run out on their own.
Arbitrage betting vs matched betting comes down to where the gain comes from. Arbitrage exploits price gaps between books and needs no offer at all, only speed, spare funds and many accounts. Matched betting needs no gap, since the free bet carries the value, and it works at quite ordinary prices. Both end the same way: once a book sees a pattern of one sided wins, the account is limited.
Asked about this
Why read the offer terms before placing anything?
One missed line in the terms can void the free bet or change how it pays, costing more than the whole offer is worth.
What does the qualifying bet in matched betting cost?
A back bet with the book and a lay at a close price on the exchange usually lose a small fraction of each unit staked, which unlocks the free bet.
Do books restrict matched betting accounts?
Many do. An account that only ever takes offers and never bets otherwise is often barred from future promotions, which ends most runs.
What happens if a back bet is voided but the lay stands?
The exchange side is no longer covered, so the lay can win or lose on its own and a planned small gain becomes a real loss.