He can't control the energy once it exits!

"Hmm, I suspected as much," Nezu remarked as he walked up to Izuku, who stood and dusted himself off, thanking all that was holy that he was durable enough that the blast had only hurt a little, instead of seriously harming him. He also idly wondered what the hell the UA Gym uniforms were made of, given his was still perfectly intact, just a bit scorched. "And that's why we start slow."

"Yeah..." Izuku remarked, grimacing.

"Did you feel anything in particular when the energy exited your body?" Nezu questioned, and Izuku turned a look towards him.

"...the explosion, obviously, but I assume you meant something else..." Izuku couldn't resist a bit of snark, before thinking back. "...not really, no. It's like... Once it exited my body, I became disconnected from it entirely."

"I see. Then it's pretty conclusive that you can only control the energy while it's inside you. You can't manipulate it any further once it's out. Pity, but not unexpected," Nezu nodded. "Unlike many Emitter Types, where at least some degree of control can be maintained, yours is like pulling the trigger to a gun when you use it like that. Whatever you did with the energy prior to the release, once the release happens, it's committed."

Izuku winced at that. He wasn't sure he liked the comparison of his Quirk to a gun, but he couldn't deny the accuracy of the comparison. "...so no building up the energy outside of my body, and holding it like that. It has to be channeled within, and then released all at once... There goes the idea of projecting **** fields or flight, but at least energy blasts aren't off the table, I just have to do them right..."

Nezu nodded, and stepped back to let Izuku try again once Recovery Girl has huffily taken a moment to look him over. "Heh, sorry, Recovery Girl," Izuku chuckled nervously. "But if I can expand my options, that will mean less injuries from me overtaxing myself in the future, right?"

"...I suppose so," she grumbled. "Just be more careful!" She strode right back to the group of teachers, muttering to herself about reckless whippersnappers.

Izuku stood in the stance again, extended his arm, and gathered five percent in the arm once more. Instead of trying to push it out steadily though, he built it up, and then, rapidly ejected the energy, envisioning heat building up in the microwave, THEN opening it. Immediately, a ball of sparking green power rushed out, and rushed towards the cement wall Cementoss had produced. Boom! The blast cratered into the wall, and left a hole midway through.

Cementoss whistled, impressed. "That's good. Packs a punch..." Izuku however looked at his hand thoughtfully.

"...hmmm... But if I punched the wall with the same percentage, I would have left a bigger crater... So it's still more efficient to just keep the energy in my body, save for the range advantage..." he mused to himself. But then again... He didn't feel the same degree of strain just holding the energy in the limb without doing anything with it. So... In theory, this could be a safer way to use a higher percentage of One For All without as much backlash.