Top left: Weighted shoulder extension - where the arms are raised as high as possible and pushed backwards under tension... This will help build strength through the transition and can be done for reps, or for timed holds.
Bottom left: Weighted shoulder extension stretch - an excellent stretch for improving not just shoulder extension, but hamstring flexibility as well. This works best as a timed hold, paired with weighted shoulder shoulder extension.
Right: A deep Muscle Up transition - Ali is showing a decent (although not quite full) range of shoulder extension. Her shoulders and shoulder blades are pressing away from her ears, giving her very stable shoulders, and her body is locked in to a solid hollow position.