I am asking myself that question too ? I thought I new a little about hydraulic lifters , at least the traditional ones used in American made cars .

Short & simple version . The lifter has the ability to " pump up " with pressurized engine oil , until all the slack in the system is taken up . At that point the trapped oil becomes virtually incompressable . Pretty much acting like a solid lifter . But unilke solid lifters , no manual adjustment is needed .

As parts wear & more clearance / slack develops , the hydraulic lifter continuse to take care if this slack & all is well .

God bless
Wyr