What is “jQuery.event.special”?
$.event.special
The jQuery special events API is a fairly flexible system by which you can specify bind and unbind hooks as well as default actions for custom events. In using this API, you can create custom events that do more than just execute bound event handlers when triggered--these "special" events can modify the event object passed to event handlers, trigger other entirely different events, or execute complex setup and teardown code when event handlers are bound to or unbound from elements.
The jQuery special event hooks are a set of per-event-name functions and properties that allow code to control the behavior of event processing within jQuery. The mechanism is similar to fixHooks in that the special event information is stored injQuery.event.special.NAME, where NAME is the name of the special event. Event names are case sensitive.
As with fixHooks, the special event hooks design assumes it will be very rare that two unrelated pieces of code want to process the same event name. Special event authors who need to modify events with existing hooks will need to take precautions to avoid introducing unwanted side-effects by clobbering those hooks
bindType: // the event you want to bind with
delegateType: // the event you want to delegate with
When defined bindType: String, delegateType: String, these string properties specify that a special event should be handled like another event type until the event is delivered. The bindType is used if the event is attached directly, and the delegateType is used for delegated events. These types are generally DOM event types, and should not be a special event themselves.
WHAT is the use of this line?
I'm assuming you meant the first line of code in your question. It basically defines bsTransitionEnd as an alias for the transition end event (the transition end event may vary from browser to browser - that is what the function transitionEnd() does : determine the proper transition end event for the browser. I've used webkitTransitionEnd here on, but it could be something else depending on the browser)
Why use an alias? It insulates any handlers that Bootstrap attaches using this alias (e.g. $('myBootstrapDialog').on('bsTransitionEnd', Bootstrap's handler) from any $('myBootstrapDialog').off('webkitTransitionEnd') that other code (say, your code or maybe another library) does - so the Bootstrap transition end animations would still work!
Why would you or another library do that? The webkitTransitionEnd is a standard event, so let's say you decide to add a transition end animation to a bootstrap dialog - you'd probably do $('myBootstrapDialog').on('webkitTransitionEnd', your handler) and later on you decide to remove the handler you should be going $('myBootstrapDialog').off('webkitTransitionEnd', your handler), but you miscode it as $('myBootstrapDialog').off('webkitTransitionEnd') - this removes all transition end events :-(.
But since Bootstrap attached it's handlers using 'bsTransitionEnd', the only way you could mess up bootstrap would be to do $('myBootstrapDialog').off('bsTransitionEnd') - not something you would do accidentally :-). So voila! Gone be the bugs where Bootstrap inexplicably stops working because of some small miscoding on your part.
The bindType and delegateType basically state that bsTransitionEnd is an alias for transition events attached directly, and ones that are delegated (bubbles). The handle is basically a filter function - all the triggered events basically go through this before the attached Bootstrap event handlers are called (if at all they are)
what is $.event.special?
I'm sure you'd know most of it already - it's a way to hook into jQuery's event handling mechanism allowing you do large scale magic like do X on every attached click event on the page (imagine doing that one by one, at each and every place you've attached an onclick event), define your own events (with all the bubbly goodness and all which comes with it), hook in and spoof events by modifying the event object, etc.
WHAT is its common usage?
I assume this was rhetorical :-) - you already have a couple of really good examples in the Ben Alman blog post you linked to
(paraphrasing) - let's say you do an AJAX submit and want to disable all clicks on the page (you probably don't want the user clicking on and navigating off to some other page via a menu, or changing a checkbox, etc.) and $.event.special.click should help you (of course it might be just easier / traditional to just overlay a transparent / partially transparent div with a Submitting... animation or something or not doing anything - after all, most users wait around to make sure a submit was successful, at least the normal ones :-))
Another use case is the one you saw in the bootstrap code, but like you mentioned, you usually don't have to go in and use this unless you're writing a library or something that you intend to distribute publicly.