When you do something without pausing to think it through, you do it impulsively. You impulsively volunteered to be in charge of the decorations for the school dance, not realizing that you'd have to blow up two thousand balloons.
When you act using your instincts or emotions, you act impulsively. Sometimes it's fun to do things impulsively, without contemplation or deliberation. You'll probably be glad that you impulsively joined the art club at school or that you impulsively got up early to watch the sunrise. In other cases, acting impulsively makes you wish you'd paused to think about the consequences, like when you impulsively cut your own bangs or impulsively eat an entire spicy pepper to impress your friends.