Complex sentence: The device downloads information only to certain beings; it did so to Jack O'Neill and not to Teal'c; it is likely that it requires the user to have the Ancient gene but it could mean that the device is programmed not to download the knowledge into Jaffa.
Split into simpler sentences: The device downloads information only to certain beings; it did so to Jack O'Neill and not to Teal'c. Two of the main theories on the differentiation are that it requires the user to have the Ancient gene, or that the device is programmed not to download the knowledge into Jaffa.