light is a very complicated subject, to really even begin to understand how it works you have a lot of reading ahead of you, i would suggest looking more into horticulture literature and not just cannabis, pick up some maximum yeilds as well, very good info in those, now to begin to grasp the reason for the MH and HPS combo switch you need to understand what these lights do and how the plants use them, for one your typical HID lamp such as a 1000watter only offers somewhere in the lines of 350-380 usable watts, these watts are called PAR watts (photosythetically actyive radiation), now depending on your choice of application whether it be HPS or MH you need to be able to differ the 2 spectrums, MH puts off a lesser PAR wattage but the spectrum is more usable by the plant, MH give you more ideal growth at a slightly slower rate, this is why some old schoolers prefer MH throughout the entire grow, i guess as you get older you understand the value of patience for quality, now on the other hand HPS puts off a higher PAR wattage so there is indeed more usable light but the spectrum is a bit weaker then MH, thus why people say HPS is more efficient than MH, it is, HPS will give you a much accelerated growth rate and definatley do the job in my opinion, but some old schoolers will tell ya the quality of bud with a MH is much better, but to a new grower who hasnt yet tweaked his enviroment to perfection will not be able to tell this difference