Benefits & Dangers
First of all, I strongly recommend against doing a spearfishing dive without a full lungfull. I know a diver who died while trying that, because he simply started the dive with less O2 than usual, and blacked out.
In addition to the benefits of negatives already mentioned, there are two additional benefits:
1. A negative dive is a fast way to initiate the diving reflex, faster than repetitive inhale dives (in this way it is often used as a 'prep' for deeper dives)
2. According to Mayol's cryptic writings (and my own experience), doing frequent negatives can amplify your maximal diving reflex, as well as accelerate the onset of your natural diving reflex (even without negative dive stimulation).
For example, Mayol, who did many long 'hangs' around 80m, noticed that during a scientific experiment, his diving reflex was both faster to initiate, and stronger, than the 'ama' divers he was compared to, even though they had been diving for many more years. He attributed his more powerful / faster diving reflex to the repetitive crushing of his rib cage & bloodshift during the many 80m hangs.
Hanging at 80m is dangerous for other reasons, DCS included, so hopefully we can achieve the same benefits by just doing shallow negatives.
Yet, there is a great danger of lung tissue damage, so always go extremely gradually and if you experience pain or cough blood or fluid, see a doctor and take weeks to recover.
Eric Fattah
BC, Canada