The science behind polyphasic sleeping revolves around the mind's circadian rhythms. During a day cycle, the mind will enter various and distinct phases where the different brain phases result in different brain wave activity. The important part about sleeping is that the brain needs to achieve the state where it produces delta waves. It's quite possible to retrain your circadian rhythm to intentionally get into deep sleep mode, the mode where the brain produces delta waves. This is the reasoning of how a polyphasic sleep pattern is supposed to work.
It turns out that as long as we get enough good quality (delta brain wave) sleep, the 8 hour window of traditional sleep isn't strictly necessary as we can proportion the good quality sleep throughout the day.