Anybody know what hose this is?

some asian guy said:

When I'm idling, the PCV is actually pulling a vacuum. It's so weird and I want to know why it's doing that.

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You're checking the wrong end of the pcv valve. If you check the inboard end, naturally there should be a vacuum because it is connected to the intake manifold. There is a check valve to keep from pressurizing the crankcase when the turbo is spooling up. There is positive pressure inside the intake manifold developed because of the turbo.

some asian guy said:

I'm sorry I didn't mean the pcv valve, that to the manifold I 100% agree with you, but there's a pre turbo hose from the valve cover with no check valve that when I put my finger over the barb on the valve cover it pulls a vacuum.

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Basically speaking, that there is a vacuum or (negative pressure) in the crankcase is a good thing. Racers install air pumps on their engines to achieve that exact condition. The only way I see that the engine can develop a vacuum in the engine would be through the (positive crankcase ventilation) system which has already been mentioned. That the engine is able to maintain the internal vacuum speaks well of the pistons, rings and cylinders.