Yelena's hand shifts up toward his shoulder, as she starts to pull him into a hug before catching herself. She doesn't know enough about how his powers work - if it's only touching his hand that has an effect, if he has to be an active participant, or if even incidental skin contact will trigger something - and while she'd be willing to take the risk on her own account, she suspects that it would only make him more miserable if something were to happen.

So she turns the motion into a gentle squeeze of his upper arm, which feels entirely inadequate as far as reassuring gestures goes, but at least isn't going to dump any more of her trauma into his head.