User blog comment:B214/General Discussion/@comment-86.149.139.182-20161003170703/@comment-9595671-20161003181546

Actually I think his transfer ability works more like if they had the Boosted Gear, hence why his boosts reset afterwards.

Say Rias was a 100 at her base power, where 10 would make Issei reach 1024 it would instead give Rias 102400 rather than 1124.