As asking a number of rhetorical questions is not permissible, I wish to clarify the following:
There are various grades of internal, so this is a very complex question in terms of where you want to draw the line. That being said, look at the following questions.
Does external use intent to drive motion?
Does external use air pressure beyond grunting to drive motion?
Does external focus on using one's own body weight (commited straight down) as a primary generator of power?
Does external focus on training from the inside out?
Does external tie the hips and waist into one unit?
The answer to all of these is no, these are internal training features. Intent is more than just thinking im going to do this. You use intent based off sensitivity to gravity and thus loads in the body to shift what carries the weight of your body and that of the oppontent. You then have to use intent to maintain how that load is carried.Such an example is shifting a load from the arm, onto the hip, without moving the arm. This is more than a mere visualization, as you will feel the load shift, and if done for a long time in a static position, the point to which the weight has been transfered to will fatigue rather than the original point.
External uses grunting to generate power, and is how most exeternal stylists use a kiai. Conditioning based off of air (pressure) is not performed in a way which results in usuable power.
External stylists may move the waist independently of the hips. This dilutes power. Often, the waist (in conjunction with the knees) rotates to deliver power rather than opening and closing of the hips.
Does internal rely on winding up, big circular movement, or rotating the hips to generate power?
Does internal rely on sequentially chaining muscles groups together to generate power?
Does internal rely on training from the outside in?
Does internal practice focus mostly on waza?
The answer to these questions is no. Hips may open and close, but rotations are not required. If people are curious as to what is meant by open and close, i would be happy to explain. External people can adapt to use the hips in the manner, even though its not usually taught this way, but what differs is in what initiates the movement. This initiation from the middle on out, is different from the chaining from the legs on out.
Waza practice is different, wether training in a kata format or waza format. In external practice, the focus is on the technique. In internal practice, the kata or waza is means by which to practice a movement principle.
Which method results in unusual effects which do not rely on speed, timing or technique?
Internal. The key difference here, is something unusual which can't be explianed by superior timing speed or technique. This is one reason why practice with an older person who can perform this way seems counterintutive.
Which method results in unbalancing on contact, not being able to feel the opponents center of balance or take it? Which causes power to stay in you? Which causes you not to feel like you can let go? Which requires no windup to generate power? Which results in people swearing you weigh a lot more?
Internal. I'm probably channeling Dan Harden here, but when you feel stuff like that, it seems out of the ordinary.Though once you do feel it, some stories of 90lbs women or old men tossing around 200lbs young men seem plausable.
Since the OP wanted to know differences
, I would focus on those above points and explore how they result, or what it feels like to have that performed on you. For example, it is possible to "simulate" the empty jacket feeling through speed by always being ahead of the other person, however it is very tiring. If you can do it with someone who isn't using speed, and isn't fatiqued, but rather leaves you unbalanced or without feedback without sweating, then you can probably assume that something different is occuring and you might be feeling "IS".