Will the definition of local variables __block What does this mean?

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- (Void) setupHelpview {
__block UIImageView * helpImageView = (UIImageView *) [self.view viewWithTag: kHelpViewTag];
if (! helpImageView) {
UIImage * helpImage = [UIImage imageNamed: @ "detail_help.png"];
UIImageView * helpView = [[[UIImageView alloc] initWithImage: helpImage] autorelease];
helpView.center = self.view.center;
helpView.center = CGPointMake (helpView.center.x * 3, helpView.center.y);
helpView.tag = kHelpViewTag;
[Self.view insertSubview: helpView belowSubview: self.headerView];
[UIView animateWithDuration: 0.5 animations: ^ {
self.pageVC.view.center = CGPointMake (self.pageVC.view.center.x - self.pageVC.view.frame.size.width, self.pageVC.view.center.y);
helpView.center = CGPointMake (helpView.center.x / 3, helpView.center.y);
} Completion: ^ (BOOL finished) {

}];
} Else {
[UIView animateWithDuration: 0.5 animations: ^ {
self.pageVC.view.center = CGPointMake (self.pageVC.view.center.x + self.pageVC.view.frame.size.width, self.pageVC.view.center.y);
helpImageView.center = CGPointMake (helpImageView.center.x * 3, helpImageView.center.y);
} Completion: ^ (BOOL finished) {
[HelpImageView removeFromSuperview];
helpImageView = nil;

}];
}

}
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http://developer.apple.com/library/ios/ # documentation/cocoa/Conceptual/Blocks/Articles/00_Introduction.html
Documentation describing in great detail
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I remember is a piece of code that can go into.