User blog comment:B214/Volume 23 discussion blog/@comment-31764731-20170726154553/@comment-29963364-20170726182505

Issei hasn't completed (mastered) CxC yet. It's just gotten way more powerful after activating DxD-G a second time, and he's able to activate CxC without the chant

The (True) DxD-G is too much for Issei to handle, so he uses (Pseudo) DxD-G instead. It's a less powerful version of the True form, and he can only sustain it for 3 minutes.

Issei has recently learned how to transform certain parts of his CxC armor into the (True) DxD-G armor (like his arm/fist). This enables him to use the power of (True) DxD-G in the specific part he transforms, without the massive stamina drain (or the near-death coma).