User blog comment:B214/Favourite moments in High School DxD/@comment-9339850-20131106182403/@comment-9339850-20131106185420

Well Ddraig and other people like Azazel have said that the Boosted Gear is affected by the user's emotions; Issei was really angry going into that fight so im sure that mental state somewhat amplified his speed and power because anger is a powerful emotion.

To tie it into the anime, in the last episode of the second season, Issei had his explosive power boost after Azazel joked that Rias' boobs would shrink with Vali's Half-Dimension ability