The original sputnik was devised by Brazilian team members that competed in Med in the late 50s to mid 60s. It was a very complicated contraption, a big torpedo shaped ballast that slid in a tube screwed onto the mid-handle air guns of the time. The ballast was held by a quick-release clip in the float. The line was usually ~60 ft. long. The diver released the clip and sunk like a rock holding the speargun handle. When the set depth arrived, the ballast would be held by the line while slipped out of the gun. And the diver would keep on going. This sputnik was designed to dive at 25 meters... very deep at the time fins were ridiculously short. Bruno Hermanny used to win the individual world title in 61, in Italy. Note that the sputnik was retrieved by someone in the boat, not the diver himself...
As in competitions now one must retrieve one's own ballast, extra ballast like the spanish and italian pendulum, or heavy belts used by the French, divers started to use something lighter and easier to retrieve. According to the one of the masters of this technique, Bernard Salvatori (from France), you must over-ballast yourself in such way that, when you expire all the air in your lungs at the surface, you MUSTN'T sink. When you start sinking, remove 500 g and so on. It all depends very much on each individual body density, wet-suit thickness and depth that one intends to hunt. Albeit has the pendulum has the advange of being easier to be retrieved, but your belt weights must always be "tuned" to the depth you intend to be positive buoyant.
The dropable belt technique (la largable) used by the French you need maybe "tune up" the belt only when use different suits, but it's more of a pain to retrieve the whole lot.
Be aware that deep diving is always EXTREMELY DANGEROUS. although drooping weights decreases the risks somewhat. The rhythm must be slower and be done always with someone watching you from the surface.
Ted
Rio