The benefits of shorting (for the shorter) are easily known, but only if the share price goes DOWN. So when the share price goes UP, (like INSM has done for the last month) shorting has no benefits to the shorter, but can have a very beneficial effect on the share holders who are betting that the share price goes UP. This is called a SHORT SQUEEZE. Talk to your broker. Or better yet, just wait for him to call you.
Note: ALL CAPS words are important and will be on the test. Class dismissed.