Each time skittish comes to affix rotation many people repeat the same chants how it is horrible and random. (While former is true, the latter is not).
Basics of threat. 1 healing point=0.5 threat. 1 point of damage=1 threat.
Skittish is simply 80% reduction to tank player's threat generation. At base a tank does 900% increased threat. That equals to 10 times (10x) threat generated.
That means 1 damage from tank=10 threat.
Threat modifier from skittish is -80%, 0.2*10=2. So with skittish you only do 2x threat.
Basically a DPS can do up to 2x damage what tank does to a target without pulling aggro.
In reality it is slightly higher because tank self healing generates threat and because of pull aggro (more on that later).
What happens in practice though isn't pure DPS. DPS players do huge bursts in short periods where tank's threat is easily left behind and a DPS pulls aggro.
This often happens on pulls and there isn't really anything mysterious about that. People often overcome this issue with CC and/or waiting for tank to generate threat before going all out.
On bosses the threat is rarely an issue because taunt gives 200% increased (3x) threat for 3s. By using your high damage abilities on those windows every 8s you can keep aggro well above the "2x" breakpoint. This also why monks are very strong with skittish. They can taunt the statue to do 8y AoE taunt around it. Use Keg Smash and breath of fire during that and you gain enormous amounts of threat.
Now to the myths. Often I hear that skittish randomly resets threat or takes chunks of it away. This isn't true and is easily seen on threat meters and by understanding how threat works.
This myth is caused by "Pull aggro" mechanic. For a mob to switch target, the second player must be at 110% threat of the current target. So if the tank has 100 threat on a target, DPS going 105 threat will still keep the mob on tank.
But if DPS goes above 110 threat, the mob switches to the DPS and DPS is the current target. Now the tank has to get to 110*1.1 threat to gain aggro back. That is 121 threat.
This threat requirement jump is why people think aggro "resets" or "vanishes". Normally you don't see that jump because with 10x threat is is easy to snap that aggro back on you. But now that 21% jump is HUGE. And bigger the threat pool the larger it is.
So even if the DPS stops damaging the mob instantly, the tank might still not be able to get aggro back fast without taunt if threat pool is high enough.
The pull aggro mechanic is a double edged sword. It gives buffer for tank to keep aggro, but can also make it harder to get back.
I hope this clears some questions about skittish and threat and how they exactly work.