Athletic Motion Golf explains why driving maximum hand speed through impact actually diminishes clubhead speed. By using 3D motion data, they show how Tour players allow hand speed to peak early and decelerate, allowing the trail arm to rotate and transfer maximum energy to the clubhead with optimal shaft lean.
Hand speed naturally peaks around waist-high (20-24 mph) and must slow down for clubhead speed to accelerate through impact.
Tour players average 7° to 9° of shaft lean with standard irons and 0° with a driver; excessive shaft lean (14°+) causes blocks, low hooks, and loss of distance.
Instead of shoving the trail elbow across the body, rotate the trail arm outward so the elbow points toward the target ('covered throw' look).
Use the split-grip drill and position your trail thumb slightly more on top of the shaft to maximize leverage transfer to the clubhead.