The practice of vipassanā meditation emphasizes the use of sati. If there is no sati, paññā cannot function. Sati itself has many different functions. First, sati allows us to be aware of the sensations that enter our consciousness as they arise. Usually our minds are trapped by delight and aversion. When a sensation arises which produces pleasant feeling we feel happy and we like that sensation. If another type of sensation arises, one that produces unpleasant feeling, we don't like it and give in to aversion. Whenever our mind delights or is averse, or likes or dislikes anything, it gets stuck on that sensation. The mind fixes itself onto the sensation but, being temporary, in a moment the sensation has passed, becoming a past experience. Immediately there follows a new sensation, but the mind, being stuck on the sensation which just passed, does not follow the new sensation that is arising. That which has just passed becomes the past, so it is said that the mind which proliferates has fallen into the past.
Just as the mind falls into the past, it can also float off and begin projecting fantasies about the future. The mind which is not aware in the present moment is the mind which delights and feels averse. The mind, either delighting or feeling averse, must clutch onto some particular sensation. As soon as it clutches onto any particular sensation it falls into the past, even if only for a second.