The dumbbell pullover is performed lying on a bench while lowering a single dumbbell in an arc behind your head. It stretches and loads the lats and chest together, with the triceps long head and serratus assisting, which is why it appears in both back and chest programs. Lifters use it to train the lats through a deep stretch and to expand shoulder mobility under load.
Log your dumbbell pullover sets in 1RM.fit and watch your estimated 1RM update automatically as you get stronger.