I love every position that opens the slave up to me and allows for him/her to be comfortable enough not to be distracted. If I had to choose a favorite, it would in combination with strap on action. The slave would be length wise over a whipping horse with his thighs and ankles secured to each side. His balls would be behind him, in a humbler or also tied. His arms to each side over his head. This opens up not only the availability for strap on but caning and whipping. The ball exposure would be just that. Maybe I'd tease them a bit with the cane or light squeeze of my hand, just enough to remind him of the vulnerability.
