3/4 surely. Any drill that works on this principle can't allow the arms to lift "to the top" otherwise it'll fall out!
I do it with headcovers or a rolled-up towel quite regularly....it's in DL and Faldo books, see p.57 in "A swing for life".
On a slight tangent here but did anyone see him warming up on the last day of The Open?
He was making full driver swings one handed and whacking the ball 240 - 260, what a freak!
Have tried a glove under my right armpit a few times for half sings, however, my full swing is just over half so it works for full swings too for me, its a good little drill.