The method of biotinylating is considered proprietary. However, a general reference regarding the biotinylation of proteins is listed in the 'PEER REVIEWED PAPERS' section of the product page:
A reagent for covalently attaching biotin to proteins via a cleavable connector arm.
C A Mouton et al.
Archives of biochemistry and biophysics, 218(1), 101-108 (1982-10-01)