What makes the credits “fully refundable”?

The credits are fully refundable because the Eligible Employer may get a refund if the amount of the credits is more than certain federal employment taxes the Eligible Employer owes. That is, if for any calendar quarter the amount of the credits the Eligible Employer is entitled to exceeds the employer portion of the social security tax on all wages (or the employer portion of the social security tax and Medicare tax on all compensation for employers subject to RRTA) paid to all employees, then the excess is treated as an overpayment and refunded to the Eligible Employer under sections 6402(a) or 6413(a) of the Internal Revenue Code.