Paul has described the process in this podcast: The Stewart Mackenzie Indaba #39 Paul Sztorc by The Stewart Mackenzie Indaba
It uses 2 tools:
- Monitor scans blocks and see if any block breaks BIP 300 rules
- Enforcer uses
invalidateblock
RPC to reject blocks
It can be considered an external way to do user activated soft fork without preparing a UASF client by forking bitcoin core.