I thought auto focus tracking comes from the computation of image on the image sensor when I was using GF1, a MILC. The auto focus tracking was good. However when I use canon 6D, a DSLR the auto focus tracking seems to be weak. Then I came up with one question: How does auto focus tracking work in DSLRs. 6D put light on the image sensor only when the shutter is released. How come the camera can do a computation of image to track the focus on the object?
A DSLR (just as a SLR) uses focus sensors instead of the image sensor to do focusing and focus tracking.
Typically the focus sensors are placed below the mirror, and the mirror is semi-transparent to let some light through to the focus sensors, and small mirrors on the back of the mirror direct the light to the sensors.
Focus mirrors -> /\--------------------- <- incoming light
Focus sensors -> * * *
This means that a DSLR has a lot fewer focus points that a MILC, so it can have some difficulties to focus on a small object.
On the other hand the focus sensors have sensors at different heights so that they can determine in a single reading approximately where the correct focus is. Using the image sensor means that you have to take two readings with a change of focus in between to get an idea of where the correct focus is, so a DSLR has the potential to be a bit faster to focus.
There are of course several other factors that affect the focusing speed, like how much light the lens lets through, and the speed of the focus motor. It's hard to make a fair comparison between two cameras where all those factors are different.
Autofocus can be determined using the main image sensor, by employing the contrast detection method. This means that the camera utilizes the fact that adjacent pixels tend to differ a lot in contrast when the image is focused in that area. This method only works, as you've noted, when the main image sensor is exposed though.
The 6D has a mirror that only flips away when the picture is taken. It can however achieve focus before the mirror flips away by using a phase detection system. This works by using a beam splitter to split parts of the image into pairs and later compared by the phase detection sensor. By recognising patterns of intensity, the distance to the source can be calculated and focus can be locked. This also works in "tracking mode" called "AI Servo" in the Canon 6D.
(Illustration made by Cmglee taken from http://en.wikipedia.org/wiki/File:Autofocus_phase_detection.svg)
These phase detection sensors are few in numbers (in the Canon 6D they are only 11) and this means that the camera can have a hard time tracking moving objects. The phase detections sensors also have limitations regarding aperture and the 6D can't autofocus with phase detection using slower lenses than f5.6 (you can read more about it here). These are some of the reasons I can think of that might get you to think that the "AI Servo" on the 6D is weak. Note that phase detection when working correctly usually delivers very good performance regarding speed.