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The lightning node locks btc and then can trade up to that limit in the network?

Correct you can lock let’s say 0.1 BTC in and send up to that really low fees! You can also receive and rebalance your funds and then if it’s a large enough amount you can move it back to the main chain

That might work.
Can btm's use it, or does that need to be on the mainchain?
How low are the fees?