As Matt said, and that is where your left elbow belongs when supporting a heavy camera/ lens combo, like a Pentax 645 with a 300mm tele on the front of it along with or without a big flash unit.

Never had a problem advancing film or firing the shutter on any 35mm slr camera with a big flash. Right hand grips and guides the body, three fingers and palm grip the body, thumb on the advance lever and index finger on the shutter button. Left hand supports the weight and fingers do the focusing and tweek the aperture ring if needed.