Piston is Motor's sandboxed code runner. An agent uploads its files and starts a run of a plan's tasks. Each task runs in a container of its image, on a new EC2 instance of its own. Piston streams the logs, and returns structured results: each step's exit code, the parsed test reports, and the outputs.
Agents use Piston through Motor's API, at /api/piston, and the motor piston commands:
motor piston check motor piston run motor piston logs
See Motor's docs.