Разница

gistfile1.m

- (void)updateWithCompletion:(void (^)(BOOL success, NSError *))completion
{
  [API executeRequest:request completion:^(id result, NSError *error) {
    if (!result) {
      completion(NO, error);
      return;
    }
    
    [Importer importFromResult:result completion:^(BOOL result, NSError *error) {    
        if (!result) {
          completion(NO, error);
          return;
       }
       completion(YES, nil);
    }];
  }];
}
// Превращается в

- (RACSignal *)update 
{
  return [[[API executeRequest:request] map:^(id result) {
    return [Importer importFromResult:result];
  }] switchToLatest] 
}