The point is to destroy all the balls by destroying a group so that the rest will fall down.

Consider the following example:



The next ball is not a yellow ball, you should pile all the non yellow balls under the yellow ball:



Now, a yellow ball comes, connect it when the yellow ball on the top, and shoot others so that they are under the yellow balls:



Now another yellow ball comes again, connect this ball with the top yellow balls and they will be destroyed, the rest of the balls will fall down and so all the balls are cleared.