The naming convention for the instance variable prefixed by _ is now clearly stated by Apple in the "Coding Guidelines for Cocoa", after the revision of 2012-02-16, with its reason.
Make sure the name of the instance variable concisely describes the attribute stored. Usually, you should not access instance variables directly, instead you should use accessor methods (you do access instance variables directly in init and dealloc methods). To help to signal this, prefix instance variable names with an underscore (_), for example:
@implementation MyClass {
BOOL _showsTitle;
}
If you synthesize the instance variable using a declared property, specify the name of the instance variable in the @synthesize statement.
@implementation MyClass
@synthesize showsTitle=_showsTitle;
https://developer.apple.com/library/mac/#documentation/Cocoa/Conceptual/CodingGuidelines/Articles/NamingIvarsAndTypes.html#//apple_ref/doc/uid/20001284-BAJGIIJE
The lecture in iTunes U, iPhone App Development CS193p Fall 2011 taught by Paul Hegarty at Stanford University, also explains this convention.
http://itunes.apple.com/itunes-u/ipad-iphone-application-development/id473757255
I'm aware of that this question is asked quite a while ago, but I myself had the same question and wanted to share my findings.
