A single scan can be performed easily enough, you just select a Dispatcher,
enter the URL of your target and hit "Launch Scan".
High Performance
A high performance scan utilizes more than one Arachni Instance to perform the audit.
The master instance will perform the crawl and then calculate and distribute the workload amongst its slaves.
This allows scan-time to be severely decreased, by as much as n times less under ideal circumstances, where n equals the number of running instances.