[zodiac]

A fork is just a protocol upgrade that is non-backward-compatible and where both the old and new protocols have significant amount of validators (in PoW, this means miners) operating with that protocol. Since the protocols are not compatible, they have a different transaction history after the fork. Since they used to be compatible, if you had 1 BTC before the fork, after the fork, all validators still agree that you have 1 BTC, but 1 BTC on one chain is not the same as 1 BTC on the other chain, so it's equivalent to having (say) 1 BTC and 1 BCH.

Everything else is just supply and demand, ie what's the market-clearing price for 1 BTC and for 1 BCH and how does it compare to before.

[arianvanp]

Could I trade with that btc on both forks at the same time?

[brazzy]

After the fork you have 1 BTC which you can use only on the BTC chain, and 1 BTH which you can use only on the BTH chain. And these are completely separate.

[zodiac]

Think about it using my definition of fork, the answer should be clear

[veli_joza]

I wouldn't use expression 'protocol upgrade', that implies it's mandatory and that current protocol is rendered obsolete. I think 'fork' better explains the mechanics.