Past and future have a feeling tone that keeps the lid on the happiness that is bubbling up below.
As soon as thoughts of past and future are dropped a shift of body tone is felt.
A tension on the face dissolves. Eyes expand as the outer edges of eyes and corners of the mouth smile slightly. Happiness flows.
Mindfulness gives you the ability to sense what is happening within.
Concentration gives you focus and attention where to direct mindfulness.
Sensations gives you the target for focusing.
Using these three tools creates feedback by monitoring the reward of internal happiness when past and future thoughts are dropped.