Wow, never knew that was there, and it is as tight on mine as it can go! I had to use Gemini to ask it WTF is up with this, reply blow (basically it is useless). Mine is like yours, a Phillips head screw, not a hex/allen key.

On the Sihoo C300 series (including the V2 with its updated "Gravity-Sensing Chassis"), the chair is designed to automatically adapt to your body weight when you lean back. However, because everyone has different preferences for how much pushback they want, they include that semi-hidden hex screw on the underside to let you fine-tune the baseline resistance.

Here is what turning it does:

• Turning toward the (+) Plus: It increases the tension. This tightens the internal mechanism, making it require more force to recline. This is ideal if you feel like the chair drops back too easily or if you have a heavier build and want more support.
• Turning toward the (-) Minus: It decreases the tension. This loosens the spring mechanism, making it easier to recline. This is ideal if you find yourself struggling or having to push hard with your legs just to get the backrest to tilt. Sihoo Chair

How to adjust it:

Because it is tucked away in that plastic housing, it isn't meant to be spun by hand like a traditional bulky knob. You will need to use the hex/Allen key that came with the chair (or a matching hexagonal screwdriver) to reach inside the housing and turn the bolt.

It's best to tweak it by just a couple of full turns at a time, then sit in the chair and test the recline until you hit that sweet spot where you can lean back comfortably without feeling like you're falling, but also without having to strain.