“Proper dumbbell flyes should be performed with the thumbs in. Thumbs in with your arms bent to, approximately, a 30-degree angle. It really makes a difference in building up your chest. When your arms are extended above your chest to begin the movement, your palms should be facing your feet so that your thumbs are side by side. Your grip should be off-center so that your thumb and index finger are touching the plates at the top of the dumbbell. I also recommend an offset dumbbell if you can assemble one. In other words, put the plates, jammed against your thumb and have the remainder or lower part of the dumbbell hanging down towards the ground, thereby providing resistance throughout a greater range of motion.
Typical dumbbell flyes — like close-grip bench presses — start out difficult and then get progressively easier towards the top or finish of the movement. This means that there is an imbalance in the resistance. But when flyes are performed my way, with the thumbs in, the resistance is constant and difficult all the way up. I call them offset flyes. And it’s important that your elbows be bent to a 30-degree angle, and that you keep your arms bent in this position throughout the entire duration of your set. If I were to tell someone how to do the exercise, I’d tell them to do it as if their arms were in a cast at about a 30-degree angle, and then keep that degree of angle all the way throughout the movement. And offset your plates. In fact, if I really wanted to get a good workout, I’d put two or three plates on the inside and about five on the outside to keep a real angle of resistance on my pecs via an offset grip.” – Steve Reeves, Building the Classic Physique: The Natural Way, Pg. 153-154