What is float? The lifter supplies the motion from the cam to the remainder of the valvetrain. If the opening NEGATIVE accelerations are too high, the resulting motion is valve loft, and can be desirable in some situations. Valve bounce can occur as a result of valve loft, or because you are trying to run a lobe faster than it was designed for with a given spring package. Sometimes valve bounce will get worse as you increase spring load due to valvetrain deflection. The valve spring is the only member returning force to the camlobe. Remember Newton? F=ma ? Basically, the spring is there to supply the F, the valvetrain mass is the m (although you can't simply add all of the masses up because the pushrod and lifter are on the other side of the rocker arm) and the a (acceleration) comes from the cam profile and is the main design focus of the designer. (Jerk, "snap, crackle and pop" make for more elegant acceleration curves) Saying that valvetrain no-follow (bounce, float, loft or whatever metric you wish to use) is only defined by valve bouncing misses some of the more important issues. All motion is initiated at the lifter foot, and gaps in any of the valvetrain linkage will result in undesirable valve motion.