Copied directly from the Help/FAQ:
"Whistle points may also be earned by flagging reviews as being abusive. Users who are logged in to Newgrounds.com will see the following text below each review: "0/0 people found this review helpful. Did you? [ Yes / No / Abusive ]". When enough users flag a review as "abusive" it is marked for our review. If the review violates our review guidelines, it will be deleted and users who flagged the submission will be rewarded with positive whistle points. If we clear the review those who flagged it will receive negative whistle points."
If you receive negative whistle points, your whistle status will go down once it reaches a certain point.
If you receive positive whistle points, your whistle status will go up once you hit a certain amount of points.
You cannot see how many whistle points you have.
These flashes may help.