Seated Floor Press / The Z Press: Advanced Overhead Pressing
Standing press allows the legs to stabilize the trunk, especially via a wider base, the Z press is performed sitting flat on the floor.
The knees are to be kept straight, and a slouch is strictly prohibited. If you try holding that position without any weight on the floor, you'll get an idea how uncomfortable and just plain hard it is to simply maintain the position.
In short, you'll need a whole lot of trunk strength, hip flexor mobility, hamstring flexibility, and lumbar and thoracic spine health to perform these bad boys well. And if you don't have every one of those things in check, the lift will suffer.
Remember, you have absolutely nothing to lean back against, and you have nothing to drive into since your feet aren't planted on the ground. That makes it one of those "no cheating" exercises similar to the guillotine press. Note that the emphasis is still on getting the head and chest "through the window" rather than overarching to compensate. Staying tall is key.