The idea is simple, if all the liquidity is on your side, lower the fee and vice versa.

Routed payments pay only on the outgoing side, so if the channel is completely on your side, the routing is almost free. If it's completely on the other side, the fees are bumped to a specified maximum.

I made the most simple version myself, which sets the fee in a linear ratio based on liquidity balance:

https://github.com/mutatrum/lnd_scalefee

This one is a bit more sophisticated, where you can have rulesets:

https://github.com/accumulator/charge-lnd

I set my max fee to what my fixed channel fee was before. So per routed sat, I only get approx. half of the fees, but the routing volume increased quite a bit and my total earned fees stayed roughly the same.