I can field this one!
The Time shift during clouds basically means that you can delay when this illumination channel starts dimming for a cloud.
So if you had Illumination One set with no shift, it will start dimming immediately when a cloud starts.
If Illumination two has a Time shift of 1 second, then it will begin its cloud 1 second AFTER Illumination 1.
This creates neat effects if you use multiple banks of LED's or T5's of different colours, or in a left to right configuration.
if its left to right you can have a cloud roll over the tank from left to right by delaying the cloud start.
If its different colours You can have your White lights dim first, then your Pink or Blues. This makes the visual colour of the tank change during a cloud instead of dimming everything all at once.