Read http://www.learn.usa.canon.com …ckbutton_af_article.shtml
and see if that helps you understand why and how you use BBF.
I don't have T2i so can't give specific feedback about using BBF on that particular camera... But over the years I've used BBF on about a dozen different Canon models and it's one of the features that keeps me a Canon shooter (I really dunno which other camera makes, if any, can do it.)
Essentially there are two slightly different Canon control layouts one might want to use BBF on: One has an "AF-On" button and the other doesn't. It appears the T2i doesn't.
When the camera doesn't have an AF-On button, to do Back Button Focusing you have to reassign the function of the * button. The * button is usually used for AE-Lock.
So let's first look at what AE-Lock is and does. You can read up on it in your manual or elsewhere, but basically it's just what the name implies... You use it to "lock" in an auto exposure (any of the auto modes: Av, Tv, P, etc.). Let me give a quick example where it might be used... Say you're photographing someone on a beach and the background is a bright sunset, which covers a lot of the image area and will strongly influence the camera's metering and auto exposure system. The result would be that the person's face would be under-exposed. This feature allows you to walk up to the subject and meter off their face or body, press the AE-Lock button and the camera will set and use that setting for 15 or 16 seconds, usually enough time to finish taking the shot. Alternatively, with an off-center subject you could spot meter their face, press AE-Lock, then recompose and take the shot.
So AE-Lock is an occasionally useful feature. But there is another way to accomplish similar without using AE-Lock... Use either of the above metering methods and make a note of the settings, then simply switch the camera to M (Manual) and set both shutter and aperture yourself, to the settings you noted. This accomplishes much the same thing.
On cameras without the AF-On button you have to decide whether BBF or AE-Lock is more important to your general shooting. In most cases, BBF wins, hands down. But there might be occasional times when you want to use both, so you have Option 1 in the Custom Function.
Most of the time you likely won't want AE locked, so would choose Option 3 instead. (Note: I bet there is also Option 0, AF/AE-Lock.)
Please read the above link about using Back Button Focusing, but let me expand upon it a little...
Previous posts here and the link above make BBF sound most valuable/useful with moving subjects and AI Servo focus... That's not the only time to use it.
I use it all the time. It works equally well with One Shot focus. It also makes One Shot focus less necessary... You can just leave the camera set to AI Servo most of the time and use the back button to turn AF on and off as needed, with a press or release of your thumb. There are still times to use One Shot because it's the most accurate.
Often BBF is used with a single AF point selected, usually the center one because it's the most sensitive.
Using BBF with AI Servo you hold the button down with your thumb before, during and after taking the shot or shots... tracking the subject and concentrating on keeping the AF point on your subject/the point you want the camera to focus upon. Just keep pressure on the button as long as you want the focus to continue operating and tracking... Separately press the shutter release once or as many times as you wish to take shots while continuing to track. It takes a little practice, but quickly becomes second nature. (Personally I can't imagine using another method now.)
Occasionally you might be working with a static subject and be concerned about precision focusing, so would want to switch the camera to One Shot. Using it with BBF is very similar: Position the AF point on the subject or part of the subject you want the camera to focus upon, press the button with your thumb and let AF do it's thing. As soon as you get focus confirmation (the green LED in the viewfinder and, if enabled, the "beep"), you can release the button and focus will stay at that point so long as the subject doesn't move and you don't bump the focus ring on the lens. You can re-focus at any time with another momentary press of the button with your thumb.
Once in focus, with static subjects you can easily recompose if you don't want the subject exactly in the center (this assumes you are using the center AF point). Just keep your thumb off the button and press the shutter release as normally, to take the shot.
A half-press of the shutter release also will still activate metering and, on lenses that have it, start IS. So you still do a half press with your forefinger, at the same time you are using your thumb to operate AF (which also starts IS, if I remember correctly, but not metering).
So, I'd suggest you use Option 3 most of the time. Practice a little with it and I think you'll like it. Back Button Focusing puts you in more complete charge of focusing. It can eliminate focus errors when recomposing and trying to put the subject off center. With BBF set up I find little need for the "AF Lock" or "stop focus" function (Option 2) because I can simply lift my thumb briefly to stop focusing, such as when tracking a moving subject and something elses momentarily comes between you and the subject.
You only occasionally might need AE-Lock and still have a couple ways to still do that (swtich to Option 1 or temporarily reset the camera to M and select both aperture & shutter manually)... When you have BBF set up, AE-Lock is just not quite as convenient as it is having a dedicated button for the purpose. Personally I feel BBF is far, far more important and something I want to be able to use all the time... so that's what I set up on all my cameras.
