A kick causes the object ball to travel in a straighter path than intended (unless it was hit full ball). If you were aiming correctly for a half-ball pot and got a bad kick, you would miss the pot due to not cutting the object ball enough. You can generally hear the bad contact as it happens. In addition, the cue ball will react differently - it will throw wider than normal and with a severe kick, can visibly be seen to leave the table.
The credit for the following insight into understanding kicks must go to Ben Plummer, who is an engineer and extremely knowledgeable about most of the science regarding billiard/snooker tables.
I’m sure that you have already wondered why you seem to get far fewer kicks at your club than you see the professionals getting, playing snooker on television. At most clubs the tables are well used and the balls are old so why should the professionals, playing on clean tables with very clean balls get so many kicks? So here’s the answer that has been eluding us for so long: THE BALLS ARE TOO CLEAN.
Super clean balls have much higher friction at the point of contact and this is the reason why the balls can sometimes jump on impact.
Three Types of Kick
The ‘Chalk’ Kick
This is the worst kick that we see is and this occurs when an amount of chalk coincides with the point of contact of the two balls. You can demonstrate this yourself by putting lots of chalk on your tip and striking the cue ball fast and near to the centre, playing a stun shot on to another ball that is about 2 feet away. You will see a small circle of chalk somewhere on the cue ball. Now position this ball as an object ball with the chalk mark in the centre. Use another ball to play at a slow pace to hit the chalk mark and you will get a horrendous kick. The good news is that this type of kick is relatively rare.
The 'Mini’ Kick
Much more frequent are these smaller kicks, where the ball often doesn’t, but can, jump. They are caused either by having the already mentioned super clean balls, or by a small amount of dirt/chalk at the impact point. You can demonstrate how badly super clean balls play yourself by cleaning the balls with washing up liquid, rinsing thoroughly and then drying with a spotlessly clean dishcloth (at all times taking the utmost care not to touch the balls with your hands) and then try playing a few shots with the balls (again, without ever touching them).
A Kick Effect
The last type of kick is not really a kick but simply a kick-like effect of the cue ball impacting the object ball when the cue ball is not in contact with the bed. We will not be concerned with this as it can be avoided by the player.
When you handle the balls you deposit a small amount of grease from your hands and this is enough to lubricate the balls slightly and make for a smoother contact between them. In billiards we handle the balls much more often and as a result get very few kicks. So if you keep the balls free from dirt and lubricate them by occasionally rotating them in your hands you will help to minimize kicks. Handling the balls will not stop the relatively rare chalk kicks but it will make them less severe. Handling also reduces the build up of static charge making the balls less likely to pick up chalk and grit from the table. The No. 10 cloth is pretty good at cleaning the ball itself so there will always be a slight risk of clean patches on the two balls coinciding in the contact, but this too is relatively rare if the balls are handled enough during a game. Finally, only chalk your cue when you need to; if the stroke is a simple plain ball one you probably will have enough chalk on your cue already.