My understanding is that the phase detection hardware on the T4i lets auto focus work at all during video. Contrast based detection is typically unsuitable for video since it requires guessing and checking until the ideal contrast is found. Phase detection works based on phases of light entering the sensor and doesn't require as much guess and check process in theory. I have not, however, seen the T4i in practice to know how the performance compares to a dedicated video camera.
In general principal however, contrast based AF is not suitable for video where as Phase Detect can be used for to make the results more passable by narrowing the range that contrast detection has to be used over.