Thought to feeling to physical reaction.

A  triggers a thought which in turn triggers a feeling and then the body responds to this feelig.

Imagine you are at a friends place and you hear the words ‘gas leak ‘.  Your immediate thought is whether you have closed the gas at home. Thoughts of gas leak, cylinder burst fire etc. rush in succession and you have a panic attack. Your mouth feels dry , your chest constricts and you start sweating.
The stimulas can be external or internal. It could be a word or a smell or a sight. It could also be  be another thought..  Sometimes the trigger could be subtle and not explicit. sometimes you get a feeling of great affection or feel at peace and you don’t know what triggered it. in many cases you might have heard or smelled something but not at a cognitive level.It might not have registerd at a conscious level but it was there.
Knowledge of this sequence of trigger to thought to feeling is important as the feelings combined with the beliefs will trigger action.Since everything happens near instantaneously we are not able to control our reactions which invariably end up badly. on the other hand if we are rehearsing an upcoming encounter , there is a chance, ( a very slim one but still a chance ) that better sense will prevail and you may modify how you are going to respond to the immenent negative encounter.
Knowing that there are two other events that have to happen before you react can be effectively used to convert our robotic responses to considered responses. Almost all negative encounters happen due to uncontrolled reactions of one sort or other. The ability to rationally consider a seeming provocation and give a rational response can  literally change your life.
Reactions are not part of the frontal cortex . They seem to be an extension of our survival instincts. They are a part of our security and are needed. But other learned reactions that are not directly linked to the survival of the organism are a luxury we can ill afford.We need to be alert to cut this chain in time before it becomes an action.
In spite of resolving not to rect to a situation or person in a particular way, we seem to be unable to stop ourselves from reacting the same way again and again. The brain seems to have devised heuristic models or shortcuts to handle certain situations and tese seem to be totally outside our control.The reactions and decisions are involuntary and instantaneous.  Till we change ie replace the present beliefs by new beliefs this pattern cannot be changed. Becoming mindful through meditation is the only way to do this. It is a long and difficult process which needs resolution  and commitment.