The full answer
HMS certification uses trained Muslim slaughterers.
For consumers who want the HMS standard, the practical answer is straightforward: choose meat from an HMS-approved source where the slaughter process and slaughterers have been verified.
HMS slaughterhouse agreements specifically require trained Muslim slaughterers meeting HMS standards.
Answers in the Halal Knowledge Center reflect the standards followed by Halal Monitoring Services (HMS), which operates as part of the Shariah Board of America. HMS certifies only what it checks.