Rabbi Yehudah replied, "He is liable." He added, "It is not the same as a hand." Rabbi Yehudah was anticipating a question of why putting it in his hand does not make him liable, but putting it on his shoulder does."
Rabbi Chiya (son of Rabbi Yehudah) addressed himself to Rav, "Son of scholars! When Rabbi Yehudah is teaching another area of law, do not distract him with questions about Shabbat. What if he does answer right? It might prove an embarrassment. Here, however, he answered correctly." His hand, unlike his body, is not at rest. So, it never performs the start of the first part of the Shabbat violation.