What is Bitcoin UASF?

User-activated soft fork
What is it? A user-activated soft fork (UASF) is a controversial idea that explores how a blockchain might add an upgrade that is not directly supported by those who provide the network’s hashing power.
The idea with UASF is that instead of waiting for a threshold of support from mining pools, the power to activate a soft fork goes to the exchanges, wallets and businesses who are running full nodes. (In bitcoin, a full node, even if it is not a mining node, is still responsible for validating blocks.)
What happens? The majority of major exchanges would need to publicly support the change before it could be written into a new version of code. After that, the new software (which has an activation point in the future) gets installed on nodes that want to participate in the soft fork.
What can go wrong? This method requires a much longer lead time to work than a hash-power-triggered soft fork. In fact, it’s believed it may take as long as a year or more to write the code and get everyone ready.
Further, if the majority of miners end up not ‘falling in line’ and activating the new rules, they could use their overwhelming hash power to split the network.
Currently this idea is theoretical and has not been implemented.