I pulled this out of AMT for you. Even though they are talking of fullscale, some of the main principles apply here.

Vertical Corrections
A vertical vibration is a result of unequal lift produced by the main rotor blades. There are two primary adjustments utilized to correct for a vertical vibration. The first is the main rotor pitch change links (PCL). The PCL controls the angle of each main rotor blade individually. To move a blade up or down, lengthen or shorten the PCL for that blade. This adjustment is typically used to correct for the vertical blade separation on the ground and the vertical vibration at hover. The second adjustment available is the main rotor trim tab. To make a blade fly higher or lower, move the trim tab up or down. The adjustment of trim tabs is primarily used for in-flight corrections. Trim tabs are very sensitive, and it is important to use care and caution when making adjustments. One point to consider when making track adjustments is that as you change a blade path, you will either increase or decrease the amount of that blade's drag. This change in drag has the tendency to move the blade forward (lead) or aft (lag) of its original position. As the blade pitch is increased, drag is increased, causing the blade to lag. If the blade pitch is decreased, drag is decreased, causing the blade to lead. This lead/lag effect results in a change in the lateral balance of the rotor disk. Thus the adage, "track affects balance" is realized.