Bookmarklets have a random number embedded in them that is generated locally using JavaScript, and then that number is embedded into the bookmarklet's code (also with Javascript). 

A hash is then created of that (which is also salted with your username) and sent to LastPass as a way to grab an encrypted copy of your key (encrypted with the locally created random number). Your actual key can then be decrypted from a login session with LastPass.com + the local 256-bit random number. This allows LastPass to offer this functionality while staying true to the privacy statement of never having access to your sensitive data.

This bookmarklet should be somewhat protected - never send it to LastPass - if combined with a LastPass login session it gets your key, so for security purposes we provide a way to 'recreate' your bookmarklet, which throws away the encrypted copy of your key and recreates it with a new random number (invaliding all existing bookmarklets).