One arm on the floor forces your core to fight the twist.
Brace hard so your torso does not rotate toward the weight, keep both shoulders pinned down.
Pressing one dumbbell from the floor cuts the range at the point your upper arm touches down, shifting the work toward the mid-range and lockout where the chest and triceps grind together. With only one side loaded, the weight tries to roll your torso toward it, so staying flat is a full-body brace. It earns its place for building lockout strength and anti-rotation control without needing a bench at all.
The floor stops your upper arm at a set depth, which emphasises the press-out and lockout and limits how deep the shoulder is stretched. It is also the pressing option when no bench exists.
Knees bent with feet flat is standard and gives you a stable base to brace from. Straightening the legs makes the anti-rotation challenge even harder if you want it.
The offset load makes your core fight the twist on every rep, and each side presses without help. It doubles as trunk training and evens out a lagging arm.
