Use it when someone or something is doing exceptionally well in a short time. She's been on a tear with her sales this month, breaking all the records.
Pro Tip 2/3
Recent Success
It implies a recent period of success, not a long-term track record. The startup has been on a tear since its groundbreaking app release.
Pro Tip 3/3
Not Literal
The phrase isn't about tearing something; it's a metaphor for rapid success. The stock market's been on a tear, and my investments have doubled.