Checked Swing

[mcarr]

I have a question concerning a checked swing. When is a checked swing a strike? Is it when the barrel of the bat passes the front of the plate, or when the hands on the handle pass the front of the plate? In a game the other day, we had a batter check his swing by turning his shoulders. The barrel of the bat never passed the front of the plate, but the hands did. He called it a strike, and I was in position C, so I couldn't tell one way or another. I looked in both the rule and case book and couldn't find anythng on it. I just want to make the right call if this situation should happen again.