This sometimes overlooked/underrated exercise is a realy great exercise especially for shoulder mobility. They will help a lot for later HBP exercises later in the series's. Slightly modified they also help for later Manna exercises. I keep these in my warm up on HBP days.
To get the most out of the bench dip.
Keep the chest up. No hunching!
Try to keep the elbows in.
When you have mastered it below 90 degrees, try to move the hands a little bit closer to each other and feel the difference.
Lastly try turning your hands so that the fingers are pointing backwards. Feel a new sensation in your upper back, shoulders and a nice stretch in your biceps.