Why use a flexible belt for freediving?
The deeper you go, the greater the pressure. The air in the freediver's lungs compresses and the waistline decreases under the pressure. With a static belt (used in scuba diving, for example), the weight will start to move. With a flexible belt, the weight stays in position on the waist.
What influences ballasting?
1/ Your physiology (weight) and body mass index (weight/height). 2/ The water: seawater offers more buoyancy (you weigh more) than freshwater. 3/ Your wetsuit: depending on the thickness of your neoprene, you'll float more or less (the thicker the neoprene, the more you'll float).
Weights and dimensions
Belt with buckle (unfolded) = 141 cm x 6 cm for 0.237 kg, rubber thickness = 2mm