Please tell me about sidechains

[Jet Cash]

I keep seeing sidechains suggested as a way to expand the Bitcoin network, and I don't understand how they can be integrated into the main blockchain. This is especially true when it is suggested that they arre administered by some 3rd party. The only way that I can see for it to work is if the sidechain was based on a main chain wallet owned by the 3rd party. They would then operate a separate blockchain to administer this wallet, and transfer Bitcoins in and out of the main chain as required. They would pick up the transfer fee as a payment for managing this sidechain.

Is this feasible, or have I misunderstood things yet again?

[Amph]

they are like altcoin tied to bitcoin, bitcoin dumped for those "altcoin" are locked until you don't dump again those token in your sidechain for you bitcoin

https://www.reddit.com/r/Bitcoin/comments/32w9he/eli5_how_do_sidechains_work/

[aws]

you can read the sidechain whitepaper here: https://blockstream.com/sidechains.pdf