This is awesomely cool (for once from Canon). They are using a new proprietary sensor which embeds focusing into the sensor array itself. Claim 30% focusing and it works while doing video. This is all about making this work well as a video.
Canon 70D DSLR With Dual Pixel CMOS AF: First Impressions | Popular Photography
Traditional phase detection AF works by diverting some of the incoming light from the lens to a secondary sensor in which pairs of sensors compare the phase of the light arriving at the sensors. Once it sees what the difference in phase is, the camera knows how far it needs to move the focusing elements in the lens, tells the lens to move to the necessary location and focus is achieved. Canon’s new Dual Pixel CMOS AF works the same way, except it does so without diverting the light to a separate sensor. Instead, the imaging sensor uses the paired photodiodes separately for AF and then together when capturing an image.
Unlike the Hybrid AF system found in the T5i and the Hybrid AF II found in the SL1, the Dual Pixel CMOS AF is just as fast with one of Canon’s EF or EF-S lenses as it is with an STM lens, though the STM lens should provide less noise and vibration to the camera’s built-in microphone.
Because of the new system, focusing during Live View is now handled entirely by the phase detection system. Previous cameras like the T5i and the SL1 use phase detection for most of the focusing, but rely on contrast-based AF to lock-on. The result is a jitter just before focus is achieved. It’s not a huge deal with still photography, but for video, it can be distracting. On the whole, AF during video mode has improved substantially. We only had a few minutes with a pre-production unit, but it really does focus the way you’d expect a dedicated camcorder to.
Why did Canon bother to include the AF system from the 7D in the 70D? For fast-moving subject tracking. While the new AF system does track subjects well, for very fast moving subjects, such as a football player barreling toward the end zone at top speed, or cyclists hurdling down an incline, the traditional AF system does a better job of tracking. Also, Canon has limited the speed with which Dual Pixel CMOS AF drives the lens toward focus. Since the feature is aimed mostly at video shooters, Canon keeps the focusing action at a smooth, slightly slower rate that makes for a very pleasing look for video.