Here is a good article explaining what **UASF **is and how it can help Bitcoin move forward from the current deadlock between Bitcoin Core with SegWit and Bitcoin Unlimited with their big-block solution.
Rather than miners, it’s the Bitcoin economy — individual users, merchants, exchanges, wallet providers and other economic actors — that activates the soft fork with the software they run. At a specific point in time, the Bitcoin economy tightens up the rules of the system, activating the soft fork. From that point on, everyone simply rejects transactions and blocks that break the new rules.
Realistically, the miners would then have to follow the new rules as well, or at least take certain safety precautions to avoid accepting invalid blocks and transactions. If they don’t, they risk producing blocks the economy deems invalid and rejects. The miners would not be able spend the coins they earned in the block reward, deposit them at an exchange nor otherwise use them. For all intents and purposes, they would not have earned bitcoins at all, and instead wasted resources producing a worthless block.