It basically involves doing 3 hour core sleep and three roughly equidistant 25 minute naps. It's fairly easy to move the naps around as needed, once you're used to this.
There's also a variant of sleeping 4.5 hours, and then having two 25 minute naps.
The idea here is that each 25 minutes nap is essentially "replacing" the ~1.5 hour sleep you're giving up. There's roughly 70-110 minutes per sleep cycle, you're giving up a normal sleep cycle and then making up for it with a power nap. Your body is forced to get more efficient on grabbing the necessary deep sleep during the core sleep period.
It's supposed to take about 1 month to adapt.
I personally did this schedule for 10 days straight, without even using an alarm clock. But I started getting sick so I stopped.