They believe He was nailed through the wrists to an upright pole with no crossbeam.
Actually, the Watchtower has published both notions. I have seen drawings in the WT depicting Jesus impaled on a vertical stake, facing the sky; and I have seen drawings depicting him nailed to a stake with no crossbeam. But what they do not believe is that he was crucified. Nor do they believe that his death paid the penalty for man's sin. Nor do they believe he was resurrected in bodily form. Nor do they believe that he is God, the Son.
It is a shame, actually, because some JW's are very nice people.