How does an underscore in front of a variable in a cocoa objective-c class work?

If you use the underscore prefix for your ivars (which is nothing more than a common convention, but a useful one), then you need to do 1 extra thing so the auto-generated accessor (for the property) knows which ivar to use. Specifically, in your implementation file, your synthesize should look like this:

@synthesize missionName = _missionName;

More generically, this is:

@synthesize propertyName = _ivarName;

It's just a convention for readability, it doesn't do anything special to the compiler. You'll see people use it on private instance variables and method names. Apple actually recommends not using the underscore (if you're not being careful you could override something in your superclass), but you shouldn't feel bad about ignoring that advice. :)

The only useful purpose I have seen is to differentiate between local variables and member variables as stated above, but it is not a necessary convention. When paired with a @property, it increases verbosity of synthesize statements – @synthesize missionName = _missionName;, and is ugly everywhere.

Instead of using the underscore, just use descriptive variable names within methods that do not conflict. When they must conflict, the variable name within the method should suffer an underscore, not the member variable that may be used by multiple methods. The only common place this is useful is in a setter or in an init method. In addition, it will make the @synthesize statement more concise.

-(void)setMyString:(NSString*)_myString
{
    myString = _myString;
}

Edit: With the latest compiler feature of auto-synthesis, I now use underscore for the ivar (on the rare occasion that I need to use an ivar to match what auto-synthesis does.