@poppoppop:
The second one does not provide as loud of a pop, and needs you to be sitting in a chair. The top of the chair's back needs to be about level with your middle back. I kind of compress my back against the chair whilst leaning back with my upper back. For me, the perfect chair to do this in is my seat in my Algebra I class. I do not recommend popping your back this way, and have been trying to figure a new way to pop my back.
I can vouch for this method… I used to do the same thing back in high school, but have been sadly unable to find any good chairs for doing this since I graduated. This method has sometimes gone sour on me though: occasionally instead of cracking, it feels more like a "creak", and is quite painful. I'm also searching for a better method of cracking my upper back.