A recurring problem with most belt balancers is that, while they balance outputs beautifully, they're frequently susceptible to back-ups if their output isn't consumed evenly. I find in my own builds that this often happens with mines, causing one side of the mine belt to constantly be active while the other is idle, burning through half the deposit while leaving the other half.
These balancers prevent that by evenly cycling through both input lines for one output, and will continue to do so even if one half of the output belt is backed up all the way to the balancer. These are single-line only but can easily be combined with any combination of in/out multi-lane balancers by placing one on the output belts of the multi-lane rebalancer.
The book contains right/left flush and right/left turn versions of all three belt speeds.