When Bitcoin forks into a new cryptocurrency… let’s call it Bitcoin2x… everyone who owns Bitcoin1x will now also own an equal amount of Bitcoin2x. But, importantly, everyone who owns Bitcoin1x will NOT also own an equal amount of Bitcoin2x. This is because, when the fork occurs, there will be two separate blockchains.
The original Bitcoin blockchain will continue on unaltered, but a new blockchain will branch off from the original blockchain. The new blockchain will be identical to the original up until the point of the fork, but then diverge from there onwards.
So, if you own Bitcoin1x at the time of the fork, you will still own Bitcoin1x afterwards. But you will also own an equal amount of Bitcoin2x.
Importantly, though, you will NOT own twice as much cryptocurrency as you did before the fork. This is because, after the fork, there will be two different cryptocurrencies… each with its own separate blockchain.
Replay protection is a feature that prevents transactions on one blockchain from being “replayed” on another blockchain. This is important because, without replay protection, a transaction made on one blockchain could be “replayed” on another blockchain… which could lead to someone accidentally losing their cryptocurrency.
For example, let’s say that someone sends 1 BTC on the Bitcoin1x blockchain just before the fork occurs. If there was no replay protection in place, then it would be possible for someone to “replay” that same transaction on the Bitcoin2x blockchain.
The person who “replayed” the transaction would then end up with 2 BTC… one BTC on each blockchain. But the person who originally made the transaction would only end up with 1 BTC… because their transaction would have been “replayed” on another blockchain.
Replay protection is important because it prevents accidental losses like this from happening. Without replay protection, it would be very easy for someone to accidentally lose their cryptocurrency after a fork.
But with replay protection in place, people can rest assured that their transactions will only occur on one blockchain… and they won’t accidentally end up with less cryptocurrency than they started with.