Why do we need to throw an IOD after giving an IOI?

Stacking IOIs means they're higher value to you and you want them badly. There's nothing interesting about that. The only way stacking IOIs will get you anywhere is if you know for a fact, and it's well established, that you both are crazy about one another.

Having said that what do you mean exactly? Do we need to throw an IOD immediately after an IOI or just keep count?

We don't need to keep count. We can IOI and force IOIs. It all depends on the situation and how it's going. For one you should learn to reward a woman with an IOI for good behaviour and punish her with an IOD if she acts in a way you don't like. Think of it as classical conditioning. If a dog does good, it gets a treat. If a dog does bad, it gets a tap on the nose.

You don't need to do this. Wherever you got this advice is wrong.

Reward compliance, punish non-compliance. By punish I don't mean get upset, I mean neg, and feign disinterest, tease, remove attention, etc... Set boundaries and stick to them. If they keep crossing them, find a new girl.