Account credit can be gained primarily through referring friends to the WHOOP platform. When you have account credit, it will automatically be utilized before charging your credit card automatically on the monthly subscription invoices. For example, if you have $60 in account credit, your next monthly invoice for $30 on the monthly anniversary of your subscription will utilize the account credit and your credit card will not be charged. You will have $30 in remaining credit to be used automatically in the next month's invoice.
You can view any account credit you have by going to app.whoop.com and hitting Membership in the menu. There you will see a row labeled "Credit" showing any account credit.