AF modes are:
Quick Mode AF:
By far the fastest and most positive option for AF, especially in low-light situations, or with subjects that don’t have a lot of detail. It’s the only AF mode for video that uses the camera’s 9-point AF system and dedicated AF sensor. However, it does require interrupting live viewing to perform, and it cannot be used at all during actual recording.
Live Mode AF:
This AF method reads focus off the imaging sensor, so there’s no interruption of live viewing. It’s not as rapid and positive as Quick Mode AF, but in a pinch, it can be used during actual recording — although it’s best-used to pre-focus before recording begins. Live Mode AF cannot focus-track moving subjects.
Live Face Detect AF:
This method is similar to Live Mode AF, but is able to recognize human faces in a scene, and will target the nearest (or largest) face it detects to focus upon. If no faces are visible, it reverts to Live Mode, with a central white rectangle as its focus target (this can be moved around most of the picture area if desired). Again, it cannot focus-track moving subjects. Unlike when shooting still images, there is no AI Servo AF capability.
AF is always activated on the EOS 5D Mark II with the rear AF-ON button. Pressing half-way down on the shutter button will not engage AF, unlike some other EOS models.