The long and short of it is a hyduralic cam always have the lifters riding on all the lobes and always have spring pressure applied to all the lobes.This keeps the cam centered.Solid flat tappet and roller tappets have a specific clearence and only some of the lifters contact the cam at given intervals and use higher spring pressures.With the loading and unloading the cam wants to migrate forward with the rotation.This can cause unusual wear on a chain or gear drive as well as distributer/oil pump drives.Cams like small block mopars have a plate to hold the cam and don't require a button,but it a must for BBs.