Custom Pushrods / Preload

It does a few things:
In terms of rocker geometry it changes the place the pushrod cup is.. .Which means you may lose more lift due to angularity issues.
It will give you more overall lift.
It will give you more mid-lift duration but no more total.
It will open and shut the valve faster (quicker rate of lift).
It makes the lifter, pushrod, and rocker effectively lighter because the leverage is greater on the valve side of the rocker where the spring sits. conversely that leverage is less on the pushrod side so the valve spring is effectively heavier in terms of the force required to open the valve.