What does it mean to "sign off" on a modification to an experimental aircraft?

...he signs off the inspection he is stating
that the aircraft is airworthy, and thus
signing off on the installation of the seat
belts. But it is the pilot who determines
that the aircraft is in a airworthy
condition before he flies..."

For the most part a true and valid point, but I will raise one minor quibble:

In the context of US aircraft and the regulations thereof, the term "airworthy" has a specific meaning; it means that the aircraft conforms to the data in its type certificate. Both amateur-built and racing experimental aircraft operate under Special Airworthiness Certificates issued for the purpose of operating non-type-certificated aircraft. Technically speaking, since they do not have type certificates, they cannot be "airworthy." The phrase most often substituted in the regulations and elsewhere is "of a condition for safe operation."

Thanks, Bob K.