I've heard the term "gwai geuk" used in two different ways. Both of the ways do not refer to a specific technique but rather a concept that underlines particular techniques.
The first way makes it interchangeable with the "mo ying geuk". That means both terms simply refer to a very fast kick.
The second way makes it SIMILAR to "mo ying geuk", but with the "gwai geuk" having heavier emphasis on deception and the "mo ying geuk" heavier on speed. I think the key in the word "gwai" isn't specifically to translate it as "ghost" but more along the lines of "sneaky" or "crafty" like the phrase "gwai gwai seu seu" (pardon the romanization if it's incorrect).
So whereas the "mo ying geuk" is sometimes construed as having a level of deception coupled with speed, such as a flip of a fan to a person's face followed by a very fast kick to the torso etc... the "gwai geuk" could refer to maybe a very well placed, and deceptively aimed kick to the knee/inner thigh of a person while engaging in upper body combat.
I know my interpretation is very nuanced but let me try one more time to clarify in case I've only made things more confusing.
The "Mo Ying Geuk" translates literally to "No Shadow Foot", but obviously we mean "No Shadow Kick". Something which is said to have no shadow while moving can essentially be said to be moving extremely fast (
arguably a ghost casts no shadow; hence I mentioned one way to interpret the two phrases is that they are interchangeable, but I'm not discussing that point because it's self explanatory). However, nothing moves so fast as to actually cast no shadow... thus we rely on illusion to create effect of super speed i.e. fan opening, crane spreading wings, cheung saam flipping etc followed by a kick to make it seem like it moved from ground level to whatever location at some "shadowless speed".
The "Gwai Geuk" can be translated to literally mean "Ghost Foot", but we obviously mean to say "kick" and therefore it can be translated as "Ghost Kick" OR "Sneaky/Deceptive Kick". From the name alone, we can already deduce that SPEED is not the key factor in this technique. Whereas the "Mo Ying Geuk" is intended to create the illusion of a super fast kick that casts no shadow, the "Gwai Geuk" is simply intended to kick in a deceptive manner. This can range from occupying the attention at the top while kicking the bottom or, perhaps, changing the angle of a kick so that it seems to be aimed at one level (high, mid, low), but then shifts and contacts a different area.
This is my understanding of the two terms. Either I'm completely off base or I'm a genius. There is no middle ground for me
