You will generally use this feature when you've added/replaced an athlete from the backend or team members have overlooked the initial email request to sign their waiver.
If at any point you need to request a waiver, go to Participants -> Athletes
You have 2 options:
You can send a mass reminder to everyone missing a waiver by clicking "Request Missing Waiver"
You can send a request to a specific athlete by clicking "Send Waiver Request" in the athlete record.