Adolphin, you are incorrectly confusing several terms and physiological effects. I think it would be worth of reading some freediving book containing basic info about the physiology and freediving theory - that would help you avoiding such accidents like you had.
Samba or correctly LMC (Loss of Motor Control) is a neurological failure due to hypoxemia. The shaking is not necessarily the same as convulsions, contractions, or spasms trying to deliver oxygen to the brain.
And what you experienced after you breath-up was a blackout (or a state close to it) due to brain hypoxia induced through hyperventilation (and accelerated by the pressure change) and not because of relaxation and low heart rate. When you hyperventilate (it means breathing (even slightly) faster or deeper than normally, lowering so the CO2 level in your blood), the body reacts to the low CO2 level by vasoconstriction of the carotids supplying the brain, and the higher blood acidity (due to the CO2 excess) causes stronger binding of oxygen to hemoglobin, which also means the oxygen cannot be easily released where needed (in brain in this case). So paradoxically, after hyperventilating, you can easily blackout due to brain hypoxia.
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