To open a LN channel, both parties puts their funds in an address that can only ...

To open a LN channel, both parties puts their funds in an address that can only be spent if both parties agree (both have to sign)

But the way this swap happen trustlessly between onchain and LN channel funds it's complicated, I don't have it clear right now

If you want to get an idea how later the balances updates happen trustlessly see my other comment https://news.ycombinator.com/item?id=27451184