I am testing different folding processes at different models ... But I understand this is not exactly at B.
What is the step with the two little lines?
this fold is basicly the same as the folding of base of a simple crane.
make a crane and understand how does the paper behave, then you'll understand what happens in this diagram.
When you bring together the three big corners as indicated by the two large arrows in drawing B, the middle part of the square section begins to stick up. What you do is grab this middle corner and fold it down, then close up the structure completely.