Forks in a blockchain are ,essentially, modifications to the blockchain that can have multiple purposes. Some devs and miners will follow the new branch, and the new coin it supports (eg bitcoin gold, bitcoin cash, omg etc). The remainder continue with the legacy blockchain (btc, ether etc).
Prior (a month or so) to the implimentation of the fork, a 'snapshot' of the blockchain is taken and date is set for implimentation. Depending on the chain being forked (eg btc) those who hold those coins (eg btc) at the time of the snapshot may find their btc holdings have been mirrored by the new currency, eg as with bitcoin cash. The new currency is said to have been 'airdropped' into the new user account.
Users can then seperate the new coins into an appropriate wallet and hold, sell, trade accordingly.
From the time the snapshot is taken, futures in the new currency can be/are traded.
Airdropping creates an instant user base, one already crypto savvy, plus market cap for the new coin.
Some fail, some succeed. If you hold and cold store, its worth checking every now and then to see if you have new coins in the wallet.
Bitcoin cash is probably the best know example of a chain fork or airdropping, or currency birth or just plain old, free money.
Check Google for a more detailed and full explanation.
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