User blog comment:B214/Volume 21 info & spoilers/@comment-199.216.41.253-20160419203901/@comment-24.113.129.70-20160419205101

Remember, in vol. 17, Wukong mentioned that Issei now has both Great Red's and Ophis' power dormant within him. And that with training, he could possibly onlock either their power of "Dreams", or "Infinity".

Also, Ophis has the ability to bring out the full potential of Sacred Gears, but that can result in the host dying because they can't handle the sudden power boost.

What I think Ophis did was bring the Boosted Gears full potential out, and the power of "Infinity" that was dormant within Issei bonded with the Boosted Gear, resulting in "DxD Mode."

As for the fact that Issei's body is made from Great Red's flesh, I think that was why Issei was able to survive the sudden power boost instead of just dying. Issei hasn't unlocked Great Reds power yet, he's just really durable.