It took me an eternity to understand how this works, so I'm writing it here then you can correct me if I'm wrong. Here with the node but the same applies to the leaf

What you want is to increase the refcount for the node pointer, but getNode()->acquire() is not an option because acquire is a private member. So you need to use the RCUPtr interface to do so, as it is a friend class of the node. The solution is to first create a RCUPtr copy of the node pointer, to increase the refcount. But when the RCUPtr get out of scope and is destroyed, the refcount will be decremented again so it would be a null operation. To avoid that you are releasing the pointer from the RCUPtr first so the RCUPtr is nullified, which prevent the refcount from being decremented when the RCUPtr is destroyed since there is no pointer anymore.