Combination shots are shots that are usually avoided unless there is no other alternative. In 9 ball a combo on the 9 can easily win you the game and often gives you a chance to win a match against a better player. If you understand the interaction between the cue ball and the object ball you will greatly increase your chance of making the shot. Object ball deflection is the key to the making the combination shot. If you don't understand object ball deflection please read the lesson How Did I Miss That Easy Cut Shot. This lesson is just a logical extension of the Cut Shot lesson of which I will go over the more important parts again. When the cue ball is struck in the center without appling english the cue ball doesn't deflect off the line of aim and goes directly to it's intended target. This is show in the diagram below.
However if the cue ball contacts the object ball at an angle even without english being applied to the cue ball and the speed at contact with the object ball is great enough the object ball and cue ball will deflect away from each other along the line of aim. This is demonstrated in the diagram below where the cue ball contacts the 1 ball at the correct aim point but is deflected to the right causing the nine ball to miss the pocket to the left.
In the diagram above the 1 ball must be hit a little (1/16" to 1/4" or greater depending on how hard the cue ball is hit) to the right of the aim point to correct for the deflection as shown in the diagram below. Below I have exaggerated with the red arrow where the contact point on the 1 ball is changed to clearly show the direction of the change (to the right) from the original point of aim.
The last thing you need to know is at what cue ball speed does the cue ball cause the object ball to deflect off the line of aim. Fortunately for this type of deflection the type of stick you are using doesn't matter. A Predator cue for example will not aim any better than any other cue on a shot where no english is used. It is all strictly a matter of the speed of the stroke. For me I find this speed to be slightly greater than a soft shot on shots within 1' to 2'. If the shot is a longer shot then the speed changes to soft medium to medium. Remember it is the speed of the cue ball at contact with the object ball that determines when deflection takes place not the speed of the cue ball when it leave the tip of the cue. You should have figured out by now that if the cue ball speed is below the deflection speed you don't adjust your point of aim. A little practice on the table with a combination shot set up will quickly show you how much to adjust your aim for deflection. If you miss the shot just watch where the combo ball hits the rail. If it is hitting the rail you are not adjusting your aim enough for the deflection of the shot. The farther up the rail it hits the more you need to adjust.
It could be argued that if you shoot all of your combos softly you don't have to worry about deflection. The problem with that in 9 ball is if you miss the combo when shooting softly you often leave it set up for an even easier combo for your opponent. I normally shoot the shot at a faster speed to guarantee deflection and to make sure that I don't leave an easy 9 ball combo.