Why does the ball sometimes hop on a follow shot, and what can I do to reduce it?
Because the cue always has some elevation (because the butt must clear over the rails, and your grip hand requires some clearance if it is over the table), there is always a downward component of force on the CB that causes it to hop.
Also, an above center hits causes squirt into the table that adds to the downward component of force. Here’s a good video describing and illustrating the effect:
To reduce the amount of hop, try keeping the cue as level as possible (i.e., less elevated) at impact with the CB.
With draw shots, the squirt effect cancels some of the cue elevation effect, because they act in opposite directions; however, the CB will hop with added cue elevation. Here’s an example: HSV B.24 – Draw shot with elevated cue and hop over an obstacle ball.
Friction force between the balls at impact can also cause the CB to hop, but this is small effect (see TP B.5). Although, the effect can be exaggerated when the balls are clingy (e.g., old, dirty, and/or chalk-smudged). This effect is demonstrated in the following video: HSV B.46 – CB and OB hop and spin transfer during follow shots. See the cling/skid/kick resource page for more info.
If the CB is airborne when it hits the OB, the CB and OB can both hop after impact. There are other effects that can contribute to CB hop. If the CB is slightly larger than the OB (e.g., with an older “bar box”), the CB will hit slightly above the OB equator possibly causing both balls to hop (especially at higher speeds). For more info and demonstrations, see ball size and weight effects. A similar situation occurs if the OB is resting in a dimple or small tear in the cloth.
Another thing to be aware of whenever the CB is jumped or hopped is: If the CB hits the OB while airborne, the effective cut angle is changed slightly. In this case, you need to aim for a slightly fuller hit on the OB, especially with fast-speed force-follow shots with the CB close to the OB. For an exaggerated example of the hop overcut effect, see:
For more info, see jump shot overcut effect.
Another important effect related to ball hop is the delay of follow or draw away from the tangent line. Here are some examples: