What must crew members provide during shoving movements unless relieved by specific instructions?

During shoving movements, crew members are required to provide visual protection to ensure the safety of the operation. This means that one of the crew members must maintain a clear line of sight to the train and communicate with the engineer. Visual protection is essential in preventing accidents, as it allows crew members to signal when it is safe for the movement to continue, and they can identify any obstacles or personnel in the path of the train.

Providing visual protection also involves other safety practices such as looking out for signals and ensuring that all movements proceed under controlled circumstances. While other options like distance monitoring and audio communication can play roles in ensuring safety and effective communication, they do not replace the critical need for visual oversight during shoving operations.