Mas como essas formas de transações mantém o serviço descentralizado e se seguro?
Ao abrir um canal (acho que é isso que eu não entendi ao certo o que é), eu não estou confiando a "alguém" todas as minhas transações?
Não. Continuas a só precisar de confiar no código, uma vez que a funcionalidade e segurança de cada um desses canais é assegurada por smart contracts (e endereços multi-signature).
Para um pouco mais de detalhe, podes ver a questão 17 num dos links que o #BitcoinCore deixou acima:
Short A:
Bob is actually paying out to Carol first, and then afterwards Bob will get his money back from Alice.
Long A:
1. Carol starts the process by producing a random number ( R ) that she will keep as a temporary secret.
2. Carol then generates a hash ( H ) of R
3. Carol gives H to Alice
4. Alice constructs a special transaction that can transfer money from Alice to Bob. But this transaction is only valid if R is included. At this point the transaction is not valid due to the lack of R. Alice also gives H to Bob, and Bob knows that H is the hash of the missing component R.
5. Bob will now construct another special transaction that can transfer the money from Bob to Carol. But this transaction is also only valid if R is included. At this point the transaction is not valid since Bob does not have access R.
6. Carol wants her money, so she reveals R to Bob; thereby making the transaction valid.
7. Since Bob is already in possession of the transaction made by Alice, he can just include R and that transaction also becomes valid. Bob knows that he has been given the correct R because he can check that H is the hash of R.
8. At the same time; Bob also reveals R to Alice.
Alice can now use R as proof that she has paid Carol (R becomes the receipt)