By straightening your hand (the hand that is holding the rod), your biceps, forearms, shoulder, and trapezius become more of a support system instead of being part of the main working muscles. The brunt of the pressure moves from your arm, shoulder, and trapezius; to your back. Your back is a bigger group of muscles, more capable to handling the pressure. As for your reeling hand, it simply just spins the reel handle when the need arises. So, no issues.