During his internship with Gran Torino, Izuku developed a way to stop relying on One For All as a special move and damaging his body in the process. He learns to spread the power across his body evenly, rather that focusing his Quirk on one section at a time.
In addition to reducing the strain caused by One For All, the technique gives Izuku a much wider variety of options in combat. Since the power is spread throughout his whole body, he doesn't need to waste time re-channeling the power from one body part to another.
Full Cowl gives Izuku much greater reflexes, maneuverability, and enhanced strength. His increased mobility allows him to scale buildings with ease and clear large jumps. By studying Gran Torino and Katsuki Bakugo, Izuku learned how to propel himself through the air using the same style of movement. This allowed him to locate Tenya Iida in Hosu City by searching throughout the area very quickly.  His mobility even allowed him to outpace Hanta Sero during a race, someone whose Quirk also provides great mobility. His new capabilities left his other classmates in awe.
Izuku displays increased reflexes during a sparring match with Gran Torino where he was able to read and evade his teacher's movements. Before developing Full Cowl, Izuku was completely incapable of matching Gran Torino's speed.
When Izuku activates Full Cowl, red, vein-like lines course throughout his body that coincides with the energy flowing through him. When fully active, his body and eyes start to glow with a green, electric aura.
Izuku began using Full Cowl at 5% of One For All to prevent injuring himself. His overall limit increased to 8% after inventing One For All: Full Cowl - Shoot Style and fighting with Katsuki. Izuku can freely adjust the percentage of One For All at any time, but going over his limit will cause damage to his body.
Currently, Izuku is able to draw out 20% of One For All without breaking any bones. However, he can only maintain 20% for a limited time and it causes great strain on his body. By training with All Might, Izuku learned to draw out between 15-20% momentarily in order to utilize wind pressure for long ranged attacks. Later on, Izuku managed to increase his limit to 10-15% without any extraneous load on his body 
- ↑ My Hero Academia Manga and Anime: Chapter 48 and Episode 27.
- ↑ 2.0 2.1 My Hero Academia Manga and Anime: Chapter 51 and Episode 29.
- ↑ My Hero Academia Manga and Anime: Chapter 58 and Episode 33.
- ↑ My Hero Academia Manga and Anime: Chapter 49 and Episode 28.
- ↑ My Hero Academia Manga and Anime: Chapter 119 and Episode 61.
- ↑ My Hero Academia Manga: Chapter 172.