The purpose of the Card is to embrace feelings that occur while playing golf. Recording intensity of either pleasant or unpleasant feelings helps us not to identify with them and therefore reminds us of constant change and transitoriness of our internal state. We learn that it doesn't make sense being attached to scores or wanting good shots only. The shot in front of us is sufficient objective.
Emotional curve shows the relationship between score and our mood. The goal is to reduce fluctuation of emotional curve and to avoid extremes. It's extremely important ta stay calm and maintain neutral mood (being more general) when bad shots start appearing.