This is a pretty unknown style of sleeping, unless you're one of those people that found your way to the many online blogs about this. The most noticeable is Steve Palvina's blog.
Simply put, polyphasic sleep involves taking multiple short periods of sleep throughout the day rather than doing it in one long chunk, which happens to be the norm. There are a few different schedules you can try, but I think the most popular is the Uberman sleep schedule.
The idea is that you have 20-30 minute naps, 6 times a day and equally spread apart. That means you get your short nap in every 4 hours. When you add it all up that means you only end up sleeping 2-3 hours a night.
The way it works is that it wants your body to focus on REM sleep, as it is the most effective. When you're doing your regular long sleeps, you don't actually experience REM sleep until the end, if at all. When you change your sleeping patterns to the short 6 naps a day, you train your body to enter into the REM sleep fast. The reason you body will end up doing this is that it needs REM sleep. It just has to have it and if you're only doing 20-30 minutes at a time, it will have to figure out another way to get it.
It is estimated that it can take about a week to adjust to this type of sleeping pattern. The first week will be tough because you'll be sleep deprived and your body won't be fully adapted to deal with it.