Striking with the extended first knuckle.

Especially to the ribs (including under heart / pectoral) - the kyūsho point is Ganka (雁下 - below the goose) - see picture above

Superficially this can affect the ribs themselves (including the pain sensitive periosteum that covers them) and the sensitive intercostal muscle between the ribs, but on a deeper level the blow is said to be directed at the heart (including the pericardial sac, which covers the heart) and the lungs. Also, either due to mechanical force, or negative stimulus to the phrenic nerve (C3-5), it can also have an effect on the diaphragm.

In Yoroi-kumiuchi (鎧組打 - armoured grappling), this would also have been a point to thrust a yoroi-dōshi (鎧通し - armour dagger) if there was access through the armour, also against Ashigaru (足軽 - foot soldiers, or an unarmored enemy.

Note: – In the ‘Shūkaroku/Tōkaroku’ (拾華録) it states that: “The Ryōi Shintō ryū makes use of Kumiuchi (組討 – grappling), Atemi (当身 – striking) and Tantō (短刀 – short blade) techniques”