Hit Selecting

It's selecting your hits. Instead of constantly clicking 12 cps, we instead choose when we to click fast, and when we to only click once. Only clicking once per hit allows you to keep your movement speed as high as possible. The short answer, is that this helps you keep combos.

Clicking fast allows you to take less knockback when you get hit, and ensures that you have good hit reg.
Generally, you should always be clicking fast before you enter the fight to make sure you get the first hit if possible.

Clicking only when your opponent is off damage cooldown (0.5 seconds) allows you to maintain a high movement speed. The faster you click, the slower your player movement will be in any direction.
We can take advantage of this movement speed by starting a combo on the opponent, then clicking only once per hit to keep up with their knockback to make sure they can't get out of your hit range.