When you watch objects, don’t focus/concentrate on them because when you focus on them, you start to pay attention to the concept.
When we focus on the heartbeat, it comes with the concept that the heart is beating. So, when we are focusing, we start to follow the movement, and then we can move forward and backward, or even sideways.
The point is to step back, relax and just know. Just know, and don’t focus onto it. Left, right, up and down, all those are concepts.
When we follow the direction, it is a concept; and when we follow directions too much, we lose the reality. Some people breathe and their head goes up and down because they are following the breath; but it’s not necessary. You just need to feel the breath.
This happens when the mind pays attention to the direction, when it follows the concept.