An open channel will by default have liquidity on one side only so cannot be use to route payments both ways. A rebalance will push 50% of the liquidity to the other side so that it can be used either way.

A "rebalance" is actually a payment yourself, exactly via the route of the participants of a swap. This will move everybody's liquidity to 50/50.

We lower the fees to 0 during the rebalance otherwise the person doing the rebalance will have to incur fees for this "internal" transfer.

We increase fees as soon as we are finished.

A triangle swap with only two participants can be rebalanced via Thunder Hub or Right The Lightning (RTL) plugin.

Select the channel you opened as Outgoing and the channel that was connected to you as incoming. Set the amount to half the channel capacity or slightly lower.

Set the fees to something low like 10 sats. If it fails you know that some participants haven't lowered their fees (or haven't opened the channel properly like making it private or with incorrect capacity).

For any larger swaps you have to use the lncli command line command that allows you to specify a route via all the participants.