Apple is down almost ten percent

If you believe that your stock is undervalued relative to its risk-adjusted expected future value, it would be a good time to do a stock buyback.

Colloquially, if you think your company is going to do relatively good compared to market expectations (revenue, operating profit, net income, etc. will be better than the market fears over the long term) - then it's a good time to do a stock buyback.

Its not a silver bullet. Otherwise companies like Cisco could come back to the 500+ billion dollar market cap they had 19 years ago. There just isn’t demand for a company that expanded to its natural limits. It’s just throwing money away slowing down the decline.