I keep seeing the "internal/external" concept come up in discussions. Even this forum has divided arts into internal and external based on the old chinese paradigm. Others talk about "engines" - with different arts having differnt engines. Etc.
But I think this way of looking at things is based on an error -- it is based on the belief that there is a particular way of moving or using your body or generating power in the fight-as-a-whole. In other words, that there is one body mechanic, a "universal" way of using your body for everything you do. I submit that isn't how it works and it isn't even possible.
In fighting you are doing different TASKS at different times depending on the demands of the moment, and that for any particular TASK, there is an optimum way of using your body to accomplish that task. There isn't an internal way of doing that task, an external way of doing that task or different "engines" for doing that task -- there are, however, an optimum way of performing that task. Skill is defined by psycho-motor researchers as your ability to bring about a desired result (perform a task) with max certainty and min time and/or effort. In other words, skillful performance of a TASK by definition will use optimum body mechanics. Not internal, not external, not some unique engine, but the optimum way of doing whatever the task is. There is, for example, optimum ways of performing a hip throw, a hook kick, etc. Not an internal hook kick, and external hook kick, an internal hip throw, and external hip throw, etc. Just good and bad mechanics.
How can we judge whether the mechanics are optimum? By results -- how well you are able to perform that TASK with max certainty and min time/effort. If you don't have good results, then you don't have the optimum mechanics for that task. With that in mind, you won't judge what you are doing based on some theoretical view of how things should be done (am I living up to the WCK standard?) but on results. And you can do this across arts.
When you examine boxing or wrestling or BJJ or muay thai or any other functional combative art, you see they don't talk about some "universal" mechanic that defines their art. They look at the mechanics as being dependent upon the task.
If you begin with the question "what are the optimal body mechanics (that bring about the desired results with max certainty and min time/effort) for this task?", there's no need to talk about internal, external, engines, etc. Those things just confuse the process.