Zero Clearance valve lash ?

Hydraulic lifters are like a hydraulic cylinder. By tightening the adjuster past 0 lash, you're pushing the plunger towards the center of it's stroke. Because it has a piston and has oil inside it provides a self adjusting cushion when the cam lobe comes up to open the valve. If it's too loose it will clatter like a loose solid lifter. Too tight and there's no cushion and it would flatten your cam lobe.