A subject line has one job: to be an accurate label. Curiosity that is not paid off in the first sentence costs more than it earns, because the reader remembers being tricked.
Short helps, because phones truncate. Specific helps more. A subject that names the reader's own situation is opened; a subject that names your product is not.
Test them, but test them against replies and not against opens. Open rates have been unreliable since mail clients started fetching images on the reader's behalf.
How we do it
No open tracking and no click redirects in any message we send. Links point at your own domain, which is also why our numbers are replies and meetings rather than opens.