He knows she’s fast, she’s faster than he is. So if he’s going to act he needs to do it now, while he still has the element of surprise and she’s still reacting to having been thrown away with all his strength. Don’t worry, Natasha no one expects Illya’s strength. He’s able to literally rip cars apart and lift them and that’s not even his full strength, of course he’ll be able to throw her entire weight against a wall with only one arm.

The arm she was twisting feels kind of numb, so he uses his other shoulder to basically tackle her against the wall and try to knock the wind out of her.