[PlantedRich]

There is a really large amount of bogus information passed around so we have to do some real filtering.
Strain on motors is often mentioned but that is old thinking. As I understand "strain", it would mean the motor was called on to do more than designed. So if you have a traditional motor, it has several things that the mag drive motors we use don't have. Take a sump pump as an example. It has a fixed impeller on a shaft, connected to a large moving part made up of metal wires wrapped on a core. These windings are fed current through a set of brushes and do carry electrical power. This is a standard type motor.
When you overload and slow down this motor, the cooling is reduced. A funny electrical thing happens as wires warm. They begin to carry more current. More current produces more heating. You've got a basic death spiral and the motor burns out when the wire insulation melts.
A mag drive has no brushes and no wires on the moving part connected to the impeller. So if you overload or even stop the impeller, there is no major increase in heat, no increase in current and no death spiral. There is a slight increase in heat due to the water flow stopping but as long as the motor is setting in water, that heat can't get very hot until all the water is turned to steam and gassed off. Not really even a thought when you just cut the flow through a canister filter.
If your water going through the filter gets up over two hundred degrees so that it steams, you do have problems but the filter will be one of the smaller things to worry about!
Reason for turning down the output is that turning down the input may make the impeller cavitate and that makes noise.

 

[PlantedRich]

Hope it helps a few that still have questions. There are short answers to a lot of things but without having the background for why, I have a harder time remembering stuff. Next time you get around a small motor that has the right type frame so that you can see inside, you may notice a bunch of sparking at one end of the motor where the brushes ride on the moving part to carry the current across. That type will burn out in pretty short order if they get jammed.

 

[PlantedRich]

I've never found the need to throttle one down but logic says that it could be done any of three ways. First cutoof, second cutoff or both just a smaller amount so that the restriciton equals what you want.
Before somebody jumps up to show a different way, I might mention that there is a firm reason not to do this type action in other places ball valves might be used.
Nothing that would ever change what we are talking about but if you were dealing with a three inch water line and left a ball valve half closed for several years, it can erode and damage the edge of the ball so that it requires replacement. But that type of erosion does not come from a filter setup!