Instead of channeling his Quirk's power into one body part at a time, Izuku uses Full Cowl to spread One For All evenly throughout his entire body. This prevents him from wasting time sending his Quirk from one part of his body to the next instead of having every part of his body ready simultaneously.
Full Cowl gives Izuku much greater reflexes, maneuverability, and enhanced strength. His increased mobility allows him to scale buildings with ease, 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. 
- ↑ 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 72.