Hi there,
Most people tend to remember outstanding people who rotated with them. If you were not that memorable, chances are, your LOR is not that strong. PDs know lots of folks and will often call LOR writers especially if they know the LOR writer and are interested in the applicant. If you are worried that your LOR writer might not remember you, drop them a note and update them on your progress/application status and that you are using their letter in your application. This is a good time to thank them again for precepting you. If nothing else, they will remember that you had good manners.
njbmd