Rocker arm play

I think this is what you are meaning: There is not a lot of tension and it only comes from trapped oil in the piston that can move inside the lifter. The tension/pressure is pretty constant as the head moves up or down with gasket thickness, but the lifter's piston has a short movement/adjustment range. If the heads lift up too much, the piston can reach its upper stop and then a gap opens up between the lifter's piston, pushrod and rocker.

But side shims won't change/fix that loose situation.