And what would the difference be between the two motions?
For thrust and pierce. As used in the Chang Hon system...
As you know many in the MA do what is genericaly referred to as the 1/2 rotation punch. It starts Palm up and rotates to a palm down position . (We will ignore for this discussion pros and cons of the 1/2 rotation versus the 1/4 rotation. This is the "Piercing" action. The piercing action, Thrust and Strike are closely related.
As a side note, ask many why they rotate the fist for a punch and see what their answers are.
See page 17
http://www.bluecottagetkd.com/files/vol03.pdf
The side piercing kick uses the same rotation as the punch for the purpose of causing internal hemorrhage. (The term Hydrostatic shock is used to explain this effect
http://en.wikipedia.org/wiki/Hydrostatic_shock)
In order to be effectively used it needs to be applied to a more fluid area of the body like the abdomen as opposed to a bony area like the head.
With a padded glove covering the hand the twist becomes inneffective. It also has no use when breaking objects like boards.
As stated in the text less twist is uded for the thrust to "cut through" a vital spot. I think a better choice of words than "Cut through" could have been chosen. Perhaps "Penetrate" would be more appropriate. Again for the best effect it must be applied to asoft vital spot such as the solar plexus as opposed to a bony area like the ribs.
The term thrust is used as part of the name of various techniques Fingertip (Flat, Straight, Twin, Upset) elbow, and others denoting it's method of execution and application.
Strike is one of the most common types of attacks and has the least twist used to destroy bone or muscle, and again the name is used for the techniques i.e. Knifehand, Reverse Knifehand, backfist and more.
So, by knowing the Chang Hon name for the technique it gives you insight into the manner of execution and application if you know the concept underlying the term. although they are often "... so closely related in principle it may be difficult to make a clear distinction among them."
Hope this is clear. feel free to diagree.