I found out a lot more than I wanted to know about just how this code works as I was working (successfully...) to get it all to work with CF8 (ColdFusion 8). The logic really happens in Store, near the method "execute" and the methods which follow.
Only read requests are actually sent through the Reader object. (readResponse.)
All three of the other CRUD request-types are direct AJAX calls. add and update must return a list of records with (at least) the ID field-value, which are merged back into the store data (see realize). These use callback routines (e.g. doUpdateRecords) in Store.
The Writer object, if you use one, is really only a data-translator, but its presence is significant.
There are several objects involved (e.g. httpProxy) depending on your exact setup. All of them, in typical Ajaxian fashion, use callbacks. Fortunately, the code is well organized and easy to follow.
I wrote an "Instant CRUD" posting not long ago in this forum, and am maintaining it as I work out the kinks of using ExtJS with ColdFusion-8 at our facility. Within the source code to that script ("InstantCRUD.CFC", in the zip) are a lot of comments.