Decimating by last interpolating factor? (Multi-stage)
The idea is that you'll be saving computational steps in the filtering part of the decimation. Think about it, if you have 800 samples then downsample by a factor of 8, now you only have to low pass filter 100 samples for the first stage. If you instead tried to downsample by a factor of 2 first, then you'd have 400 samples to filter after the first stage. The order matters because of the number of computations taken in the filtering step.