It would be impossible for Wilhelm in here to harm anything in Istredd's domain which is why it is the perfect place to practice. His strength in the Horizon is impressive since so much of it requires the mind, enough to help Wanda in the midst of her power during the heralds. Even when they're on the outside, Istredd is strong enough to contain most magic from a young user. As uncontrolled as it may be to them.

"Good job, you pulled it to you. This next thing, think of it like stretching or lifting weights. Watch me."

It's an exercise, intended to help Wilhelm learn how to fluctuate the intensity of it, and also so that he might have an easier time putting it out later. These are the fundamentals, it'll be some time before the boy is confident enough to do real fire magic, but it is good to start out small.

Istredd uses his own fireball and moves one hand to on top of the flame while the other stays at the bottom, hovering it in between. "Just as you reached out and felt the fire enough to pull it toward you, now feel it between your hands. I want you to try and grasp the energy with your top hand and then pull, stretching the fire between. Make it larger, longer."

He does exactly that, the flame growing longer as if stretching like a rope between his hands and then more volume in between the palms. "And then you push down and make it smaller, hold it down as close as you can." Istredd pushes until the ball is almost invisible, as if he could snuff it out between them. "Don't worry if it goes out or gets too large, it's all under control in here."