This diagram states that arousal can be both positive and negative. When are less aroused performance is worse, and when we become to aroused pur performance is negativley effected. But is when our arousal levels are medium, that we peform best. This suggests that is a positive element but is we let it spiral out of our control it can have negative impact.
Its similar to 'caring' out your sport. If you do not care enough you will not perform to your best. Also if you care to much you may make rash decisions.