Overhead carries consist of locking out barbells, dumbbells, or kettlebells overhead. They directly force the spine into hyperextension because the overhead lever is so long. This challenges you to maintain a tight rib-to-pelvis relationship and to stabilize a good upwardly-rotated position for the scapulae.
They're a good choice for people who tend to be stuck in extension – depressed scapulae and clavicle, tonic lats and erectors, rib flare, etc.