They use the bandwidth available on the local channels' digital OTA broadcast. They do this through a partnership agreement with various local broadcasters. The FCC allocates a 6mhz bandwidth for the digital OTA channels, but the broadcasters aren't using all of that bandwidth, so USDTV is using the leftover space. If you can understand that a digital cable channel takes up significantly less space than 6mhz, then you can understand that one digital OTA channel has space to offer many digital broadcasts in that same channel space.
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