Carve the upper chest with a deep stretch.
Keep that elbow bend fixed. It is a fly, not a press.
Setting the bench to around 30 degrees points the stretch and the squeeze at the upper chest, the region flat pressing tends to leave behind. The wide arc with fixed elbows takes the pecs through a deep loaded stretch at the bottom, with the shoulders assisting to control the descent and stabilise the arms. It is the isolation move for filling out the top of the chest once your presses are doing their job.
Around 30 degrees, as set here. Steeper than 45 starts handing the work to your shoulders, flatter loses the upper-chest bias that makes the incline version worth choosing over flat flys.
Until you feel a strong stretch across the chest with the elbow bend unchanged, roughly level with your torso for most people. Sinking deeper by bending the elbows or letting the shoulders roll forward trades a good stretch for a risky one.
Usually the arc has collapsed into a press, elbows bending on the way up, or the weight is too heavy to control wide. Lock the soft elbow bend, lighten the dumbbells, and think about hugging a barrel rather than pushing the weight.
