This overhead press variation forces you to have perfect form. It will immediately expose lack of t-spine extension or exaggerated extension through the lumbar region by making you lose your balance. Overhead presses are difficult enough as is, but throw in a kneeling component and you just raised the difficulty bar several notches.
If you're capable, kneel on a bench rather than the floor. The fact that your feet can't anchor into the ground for support increases the difficulty.
To complete this move successfully, every facet of form and technique must be dialed in. Because there's little room for error, grab a spotter the first few times to make sure you don't face plant.
Start your biceps work with hammer curls, then a preacher curl variation like the one here. For safety, finish with heavy barbell curls when you're biceps are fully pumped.
It seems like you've been adding weight to the stack each time you do triceps pushdowns. The veins in your neck stick out like a garden hose as you power through the sticking point, and the cable starts cutting into your neck, just like your idol, Tom Platz. You stop, however, before it cuts through and leaves a bloody mess on the floor.