The different letters here refer to different melodies or thematic material. If you look closely, although you can see 7 letters, in reality there are only 4 options - A, B, C or D (it's quite different from these 7 letters - ABCDEFG, isn't it?). The episodes A, B, C and D have to be quite different from each other - perhaps they all have different melodies, harmonies, tempos, meters, rhythms, textures and they could be performed using different registration.
Let's take a look at this structure (ABCDCBA). What do we see here? D is in the center, A is on the outside, then comes B and finally C. In other words, CBA in the second half is like a mirror version of ABC in the first half.
Therefore, one of the terms used to describe this form is Concentric form - meaning that in the center you have an episode around which there are mirror episodes of several other themes.
We could also regard this form as having a ternary structure - ABC-D-CBA. Then we could have something like compound ternary ABA form which could be the simplest example of concentric form.
Concentric form can have 5 parts - ABCBA, or 9 parts as well - ABCDEDCBA. The main idea is the mirror version in the second half around the central section which forms a nice symmetry.
I hope this explanation made the principles behind this form more clear to you and you can apply it in your 10 minute improvisation practice.