One dumbbell overhead, deep stretch, full triceps lockout.
Keep your elbow pointing at the ceiling and tucked in, don't let it flare out.
Holding one dumbbell overhead and lowering it behind your head puts the triceps, especially the long head, under load at its longest position, which is exactly where overhead extensions beat pushdowns. Working one arm at a time keeps the strong side from stealing reps and lets you keep the upper arm truly vertical. It earns its place as the stretch-position triceps builder in your rotation, filling the gap that pressing and pushdowns leave.
The weight is too heavy or your shoulder mobility is limiting you. Drop the load, keep the elbow aimed at the ceiling, and shorten the range slightly until the position improves.
Both work. Seated with back support keeps you stricter, standing forces your core to help stabilise. If your lower back arches while standing, sit down.
Lighter than you think. The stretched position is unforgiving, so pick a weight you can lower slowly behind your head on every rep without the elbow drifting.
