Having had the slw for 2 years now, along with many other jebao poeerheads with master slave function, I dont see how you cannot figure out how to use it.
As for the different speed, picture this. 2 pumps at the same speed working in master slave, will cause a rocking wave action ,similar to a wave box does. Now if you have the same pumps in master slave, but 1 is set to slow speed, while the other is in fast. You will get 2 different actions. One a rolling wave then the next a choppy one, which will cause major turbulence. This isn't a hard concept to decipher. And both have a benefit. With these your given the ability to choose what suits YOU best.