Practice the postures individually or the set and they have different effects. The posture with both arms up is the actual appetite chi kung posture. In this posture the arms also have more of a straightness than usually found in correct posture for tai chi generally.
That said, beginners' posture is rarely that good to begin with so it's not something to over intellectualise. This goes across the board, the less one thinks about this being correct or that being right the better the chances of actually doing some chi kung.
Chi Kung is defined by the splitting of the awareness and the concentration. So no thinking at all really. The only usual postural aches and pains should be the leg or shoulder muscles. If your body is a bit messed up try my 3 Foot Chi Kung instead, it's a very cool feeling.
The posture with the arms crossed locks off your energy. Chi Kung can give the impression that there is a conflict between the mind and the awareness but it's more the case to give one's self a bit of space to practice. Then of course there's just yourself to look at things more clearly.
The third posture with one arm up is balancing maybe. I saw a guy years back cruising down the motorway at about a 100 mph with just one hand resting on the handlebars. Very relaxed, balanced feeling in view of the circumstances.
Appetite Chi Kung is a set of three postures which can be practised with the back ever so slightly more rigid as they are aiming for a structural integrity as well as the creation of an appetite. Big appetite for life on a mental level. It's a rare feeling, mostly we eat on a regular basis with set meals, not because of a raging hunger.
Chi Kung generally is not such a mystery, pretty much a case of standing still and bending the knees. Since we all have the same body, same mind, it's fair to assume anyone would get there in the end. Realistically I'd recommend lessons since that's how I learnt it. Anyone could and should be able to change a car tyre but if you don't do it right you are going in a ditch.