For a soft fork to activate miners currently are required to signal their support. For SegWit that requires 95% of blocks over a 2016 block window signalling in favour.
Currently the Bitcoin network has around 6,000 nodes. ~5,000 of those are running Bitcoin Core, and ~3,000 nodes are running Bitcoin Core versions that fully support SegWit (>=0.13.1).
Those 3,000 nodes, despite being SegWit ready, can currently do nothing more than wait for miners to activate it. This just isn’t happening, and they are being denied all the benefits that come along with it.
UASF would mean there is no signalling by miners for activation. Instead a new version of the Bitcoin software is released and installed on nodes which has an agreed activation point in the future.
At the time of writing we’re at block #455855. If we wanted to activate SegWit in 90 days time (approximately 12,960 blocks), the new version of the software could say “from block #468,815 only accept blocks that support SegWit”.
Let’s imagine that by block #468815, 60% of nodes have upgraded to this new version of the software. In addition a number of large exchanges and Bitcoin businesses have also publicly announced their support and upgraded.
This gives miners a conundrum. If they don’t create SegWit compatible blocks from that point all those nodes and businesses will completely ignore the blocks they create. This would prove very expensive.
SegWit blocks will be backwards compatible, as nodes that haven’t upgraded their software will still recognise them as valid.
This means that the odds of SegWit activation succeeding are massively stacked in its favour, as the miners are the only ones who incur an economic cost if they make an incorrect decision about activating. By far the safest option is to mine blocks compatible with SegWit as these will be accepted by 100% of nodes, rather than the 40% of non-upgraded nodes and economic minority.
So that’s the basics. The nodes basically tell the miners that if they don’t play ball they will be ignored. It would take an incredible amount of nerve (or stupidity) for a miner to refuse to mine SegWit compatible blocks when such a huge part of the network will suddenly start invalidating their work.
If just 51% of miners update then SegWit would activate without any issues.
I don’t believe SegWit would struggle to gain miner consensus, but in the unlikely event that over 51% of miner hashrate refused to switch this would create a fork in the network with non-upgraded nodes seeing one version of the blockchain, and upgraded nodes seeing another.
I believe a more likely outcome is that a rival implementation of the bitcoin software would use the activation point as a catalyst and attempt to rally support for and introduce a change of their own.
This wouldn’t be a property of UASF itself, but an activation point creates a natural window of opportunity to force the community to make a choice between rival visions.
Instead of a soft fork, a rival implementation could activate a hard fork block size increase. In addition they would likely need to change how quickly the difficulty is adjusted and hopefully add replay protection.
This would lead to all nodes and miners having a clear choice between two directions for Bitcoin. The likely reason a rival implementation wouldn’t attempt this is because they feared humiliation, but a set activation point would present a perfect opportunity to also force a referendum on the block size debate to those with the conviction to back their belief.
There would likely be just one winner, with an economically insignificant altcoin created on the other side. The block size debate would finally be settled once and for all.