The objective of the SP is to transfere as much energy of the projectile to the target to cause as much tissue damage as possible. This effect is sometimes called the hydra-shock effect.
For when soft points and hollow points expand, the damage is from the shock of suddenly opening a fluid space which can be a ripping effect. Other damage can be from secondary projectiles from the original that fly off as the bullet expands violently until energy is lost and all motion ceases.
There isn't any "buzz saw" effect as seen on TV. Most 223 rifles will have a twist rate of 12:1 up to 14:1. These numbers mean that the projectile will rotate once every 12 and 14 inches of forward motion respectively. No matter the velocity, the twist rate remains the same. If you have a target mass of eight inches thick and the projectile passes through, then it did not have a full rotation upon passing through. If the projectile did not pass through, it still did not have a full ratation since all energy stopped once the forward motion stopped.
When one uses a FMJ, much less energy is passed to the target due to its inability to transfere energy to the target. FMJ's can be known for the tumbling effect, but usually when the projectile has to travel a great distance and the bullet weight and length is too great for the twist rate. The tumbling happens before target impact and "key holes" the target. Accuracy for a tumbling projectile is crap.
Not if it only tumbles/yaws on impact, or more properly on penetration, of the target. Which is exactly what FMJ .223/5.56x45 (and "Russian" 7.62x39 and 5.45X39) do.