A User Activated Hard Fork (UAHF) is a peaceful method of dividing a blockchain through a planned change of consensus rules at a certain block height. This implies that the participants (nodes) that execute it after reaching a certain point separate from the original blockchain network and start to form a new one without this implying an attack, hash war, updating or disappearance of the previous blockchain.
It was first executed by bifurcating Bitcoin Cash (BCH) from Bitcoin (BTC) to become an independent currency with its own characteristics.
The UAHF that gave rise to Bitcoin Cash was raised as a response to the proposal called “UASF BIP148” and among the changes implemented through it were an anti-replay protection that made it safe to make BTC and BCH transactions even after the split, a increasing the size of the blocks and removing the code of the “Replace By Fee (RBF)” function in order to strengthen the security of transactions without confirmations (0-conf transactions).
These types of technical processes that occur in some cryptocurrencies can be beneficial and in a certain way help to debug some Blockchains and improve them at the same time.
Some of the most famous cryptocurrencies that came from a bifurcation are Bitcoins Cash as we mentioned before, Bitcoin Gold and Ethereum Classic.
It all depends on what the users propose and the direction they intend to take with the cryptocurrency, for example in the case of Bitcoin and Bitcoin Cash, some users were in favor of improving the system and others were in favor of following the vision of Satoshi Nakamoto, in the end both currencies managed to coexist since currently both have a community that supports them.