With my axe kick, I always throw it with a chambered leg. This is also how i teach my students to throw it. Basically, you bring your knee up as high as possible, and do a front kick as high as possible. (The higher the axe kick, the more room you have for adjustments, like if someone sidesteps ), then proceed to bring the leg straight down, or slightly to the side.... depending on the situation. I believe the bent knee axe kick is more efficient because it can be launched in close quarters combat as well. Also, the 1st half of the kick resembles a front kick, so this camoflauges the kick and enables deception. Its hard for th opponent to spot a bent knee axe-kick. As with the straight leg axe-kick, its a little bit easier to be seen. Also, the bent knee axe-kick is alot faster.... atleast for me it is.
The only problem I have experienced with the axe-kick, is once you started the movement, the action must be completed. You can't change your movement once you have commited to the axe-kick. So if someone is able to spot your axe-kick, you can be really vulnerable during this kick.:asian: