A hyposyringe was a medical device, similar to a hypospray that was used to inject a patient with medication.
He however modified the hyposyringe and fitted it with an optical chip reader that was especially modified to read data from Starfleet isolinear chips. J'Dan then extracted digital information from a computer, encoded it in the form of amino acid sequences and transfered those sequences into a fluid in the syringe, usually a deoxyribose suspension.
Then he would inject someone, sometimes even without their knowledge. This way, the encoded information would be carried in the hapless victim's bloodstream in the form of inert proteins. This ingenious invention allowed him to transfer secret schematic drawings, of Federation dilithium chambers, for example, to the Romulans. (TNG: "The Drumhead")