What is Porter's focus strategy, and when is it especially promising?

Concentrate all effort on one narrow market segment or niche — especially effective when competitors are "stuck in the middle" and excel at neither cost nor differentiation.

The focus strategy is the "third option." Instead of competing across the whole market, you pick a specific niche and serve it better than generalist competitors can — applying either cost leadership or differentiation within that narrow segment.

It's most promising precisely when rivals are spread thin and mediocre everywhere (the "stuck in the middle" trap), leaving a focused specialist room to dominate the niche.

Tip: A boutique firm specializing only in, say, automotive cyber security can out-serve a giant generalist consultancy in that one domain — that's focus.

Go deeper:

• Porter's generic strategies (Wikipedia) — the focus/niche strategy as cost leadership or differentiation applied to a narrow segment.