Adam Clarke on Acts 26:1

Then Paul stretched forth the hand - This act, as we have already seen on Act 21:40, was merely to gain attention; it was no rhetorical flourish, nor designed for one. From knowing, partly by descriptions, and partly by ancient statues, how orators and others who address a concourse of people stood, we can easily conceive the attitude of St. Paul. When the right hand was stretched out, the left remained under the cloak, which being thrown off the right shoulder, to give the arm the fuller liberty, it then rested on the left: under these circumstances, the hand could be stretched out gracefully, but was confined to no one attitude, though the third and fourth fingers were generally clenched.