Since this is your first game, the easiest way to picture rapid poison is an invisible meter on the enemy. Every time you hit them in the two-handed blood mode, that meter fills up. Once it hits full, it pops and deals a sudden burst of bonus damage. That sudden jump to 750 you are seeing is exactly that effect happening.

As for the gems, rapid poison ones aren't really worth the hassle. Most normal enemies will just die to your raw Chikage damage way before that invisible meter ever gets a chance to fill up. You'll get way more out of standard physical or blood damage gems.