Single tuner records 4 networks simultaneously?

I see from the Hopper press release that while it only has 3 tuners, the new unit will be able to "record using a single tuner all of the primetime TV programming from ABC, CBS, FOX and NBC". Meaning I guess you can then simultaneously also record two other networks.

So how can one tuner record four different networks at the same time? Sounds like an awesome feature, but I'm curious how they're accomplishing that?