I designed this kettlebell mobility combo to stretch the back/spine and work the core. The supine spinal twist combined with a kettlebell flye. The great thing is that light load is added to the opposite side of where the legs turn to, this keeps the opposite side (loaded side) closer to the ground and gives a deeper stretch. While bringing the legs up and performing the flye you also have two opposing forces which is great for the core.
The kettlebell allows you to work unilaterally and move separately, this is especially important when it comes to pressing, the shoulders are an incredibly complex joint that require extreme attention when working out, kettlebells allow you to move and adjust freely during movements, take a barbell and there is no lateral out/inwards movement or adjustments that can be made during a bad rep.