User blog comment:SpookyBoogie27/Light Novel Spoilers/@comment-33447191-20130118201121/@comment-33447191-20130124053800

I'm gonna need some more information when it comes out, until then....

Theory 1: Issei can summon his Boosted Gear, but cannot activate his primary ability i.e. doubling his power every 10 seconds or enter Balance Breaker, however, he can use the secondary ability, allowing him to transfer his power to his teammates.

Theory 2: This event was after Vritra and Fafnir help regain Ddraig's conciousness, and  Issei was able to use his Boosted Gear in correlation with Saji's Absorption Line to transfer his power to his teammates.

or... if you're talking about him being able to change any part of his body into that of a Dragon, then it's because it's not related to his Sacred Gear, it came from his new body which was crafted from Great Red's flesh and Ophis' powers.