why did will and Jim feel pain when mr. dark clenched his hands into a fists

Apparently Mr. Dark can search their minds and that is painful. Mr. Dark asks Charles questions about two boys that he says have just won a carnival prize. He opens his hands and one boy is tattooed on each palm. He wants to know if Charles knows their names. Charles lies and Mr. Dark clenches his fists; both boys feel terrible pain in their heads. He announces that he knows the boys' first names but not their last and continues to squeeze until blood drips from his palms into the drain.