Probably the side piercing kick with the heel, again not sure of the terminology you use.
Of note with "Side Kicks" is that the Chang Hon terminilogy does not just differentiate the kick, but gives insight into the application as well.
While there are more than 3 types of attacking motions in the system, the most prevelant are "Piercing" "Striking," and "Thrustiing."
Examples are Piercing : Side Piercing Kick, Front punch (The term Pierce does not appear in the name) Strike: Knife hand / Backfist Strike,Thrust: Side Thrusting Kick, fingertip thrust.
As with many things distinctions are not always "Bright Line" The side piercing kick uses the penetrating rotation to cause hydrostatic Shock and internal hemmorhage applied to softer areas of the body.
Thrust is used to "Cut Thru" ( IMO better choice of words would be "Penetrate". ) and typicaly has only little twist and is used to attack soft areas like the eyes, throat etc. (i.e. Fingertip / real elbow)
And Strike has the least twist (i.e Knifehand / Backfist ) and is used to break bone etc.
Then in Kwang Gae the first kick in the 2 consecutive kicks is a "Pressing" kick which by definition is used solely to attack the knee joint.
So, improving the sidekick in the Chang Hon System first involves understanding which one you are doing and the optimal execution depending on the application.