You are experiencing emotional symptomatic responses to the physical act you are initiating.

Physical responses and emotional triggers to those responses are a two-way street. In other words, the 'Fight or Flight' response can be initiated by visual cues or by intentional mimicing of the physical manifestations of the response. In other words, 'sadness' can be felt by tightening the diaphragm, decreasing the depth of your breathing and increasing the rapidity of breaths, and 'stuttering' or catching your breath on the exhalation just like when you were a child and you were trying to stop crying.

So, whatever you're doing physically as you are practicing your countermeasures is dredging up some sort of symptomatic responses that are 'running chills' up and down your spine.