What does it mean to enable CB when a rollback is false?

Circuit breaker is a new feature to help ECS detect that your service deployment failed and stop your service from continuously trying to restart tasks. I haven't used this feature directly but, based on documentation, if you enable circuit breaker without rollback, you will get the benefit of ECS failing fast and not thrashing your tasks until something tells it to stop trying (e.g. CloudFormation or manual intervention).

References:

• http://docs.amazonaws.cn/AmazonECS/latest/APIReference/API_DeploymentCircuitBreaker.html
• https://aws.amazon.com/blogs/containers/announcing-amazon-ecs-deployment-circuit-breaker/#:~:text=With%20this%20feature%2C%20Amazon%20ECS,the%20need%20for%20manual%20intervention.