Essentially, a TTL (Through The Lens) system has a sensor in the camera body that "reads" the amount of light that falls on it and turns the strobe off when the amount of light needed for a good exposure is reached.
actually, that's how one wishes TTL works, but it is not how it works. TTL on a camera sends low intensity fre-flash(es) and us that light to meter the scene and determine how much light the strobe really has to send for a properly exposed shot. The light is measured by a separate sensor in a DSLR that is out of the light's path when the image sensor is exposed, so the pre-flash was used.
Units like the SL-960D has those pre-flash cancellation features where it won't fire on the first 1-5 pre-flashes, but you still have to control strobe's intensity. Since the strobe suppresses the camera's pre-flash(es), the metering from the pre-flash usually causes the camera to think the image lit by the pre-flash was excessively dark, thus it often sends a full burst of light when taking the picture, resulting in shortened battery life.
New models like the SL-961D interfaces with the camera the same way, but has its own light sensor so it will measure the amount of light sent out and shut it off when it senses enough light has returned. This will only work when the strobe is at the same distance to the subject as the camera and aimed at the subject. So moving the strobe farther to the subject or doing back-lighting will result in over or underexposing. Plus, since these strobes can't communicate with the camera in regards to its aperture setting, etc like land cameras. If you change the f-stop on your camera, the strobe does not know anything about it and will go about measuring the amount of light it needs to fire off as before, thus you will get an over or underexposed exposure and you have to adjust the strobe accordingly.
On a TTL capable strobe, the strobe fires off every pre-flash that the camera also fires off instead of suppressing them, in the same duration as the internal flash. The camera uses the external's pre-flash(es) to meter the image thinking it is from its own internal flash and does the same when the actual picture is taken.
Since the external strobe short duration of pre-flash (even if it flashed off for the same duration), is much brighter than the internal flash, thus a much more light was returned from the pre-flash, the camera thinks it does not need as much light from its internal flash, thus it fires off a shorter burst than it would normally do if it was just firing off its internal flash to take the same picture, resulting in the camera's batteries lasting much longer.
With TTL enabled strobe, since the camera's meter measures the strobe's intensity, you can place the strobe at different distance to the subject, like distant side lighting, aiming with just the edge of the strobe's light path brushing on the subject, multiple TTL strobes, each with different intensity, etc. If you zoom in on your subject, the metering will only be on the zoomed in subject, same as wide angle or if you add a wide angle wet lens or use a dome port. Since the metering is determined by the camera, the multitude of metering patterns provided by the camera should also apply in such situation, e.g. single small subject in dark background...if you select spot metering on the subject, then the subject would be the only thing properly exposed.
