I think any brushing action increases time the ball remains in contact with strings. Best example that you can feel is a low volley. Try hitting a low volley with a square action where you just hit straight through the ball toward the target. Now, try hitting a low volley where you hit downward with an outside to inside pulling across motion. It will feel like the ball stays on the strings longer.
Same thing with groundstrokes. A square flat groundstroke where you hit out toward the target will not have as much string contact time as one where you pull upward and across thru contact.
Difference is small and you may already use the brushing action.
Agree with others, flexible frame, softer strings, or lower tensions also increase dwell time.