Your secret-key (associated with your other keys in your account, viewed in your account after you login to the developer's portal) is used to digitally sign certain PubNub commands, such as grant. The Secret Key is not actually passed into the grant call, rather, it is used to initialize the PubNub object. Any PubNub object initialized with the secret-key will have the ability to grant.
In case of any questions, contact us at support@pubnub.com.