As mentioned in the comments, I think it’s fine to talk about ARC and the contents of Automatic Reference Counting here.
__autoreleasing is not meant to be used like that. It’s used for passing indirect object references (NSError**, etc). See 4.3.4 Passing to an out parameter by writeback.
According to 3.2.4 Bridged casts, the __bridge_transfer is correct as the CFURLCreateStringByAddingPercentEscapes function returns a retained object (it has “create” in its name). You want ARC to take ownership of the returned object and insert a release (or autorelease in this case) to balance this out.
The __bridge cast for originalstring is correct too, you don’t want ARC to do anything special about it.