This is one of my favorite shoulder pressing variations. It's not only a great way to spice up your shoulder workouts and hit your shoulders a bit differently, but it also requires some additional core muscle engagement in order to maintain the offset load.
Although you can certainly perform this exercise using a dumbbell, do it with a kettlebell because its design really works well for this exercise.
For this intensity technique, do the concentric part of the exercise on your own, then have your training partner push down during the negative as you resist.