I apologize for the incompleteness of the explanation of the gartel in the esrog. Here is a fuller version.
At the end of the flowering stage, the petals fall off, leaving the very beginning of the fruit surrounded by a relatively tightly closed ring of filaments with anthers at the top of the filamnts. As the fruit expands, this ring of filaments breaks apart and the filaments and anthers spread out to allow more room for the fruit to grow. Occasionally, more in certain esrog varieties and less in others, the filament ring does not break apart so easily, and for a while will pinch the fruit, forming the gartel. Interestingly, the esrog is unique among citrus fruits in that the final shape of the esrog is unidentifiable at the beginning of the growing stage, whereas in all the others, the shape is easily recognized from early on.
It was once proposed that the gartel is caused by a virus, but the proponent of that theory has since retracted his opinion.
This is not my field of expertise. The source of the information above is a talmid chochom and yarei shamayim who makes his living as a full professor and Chief of Agricultural Research at the Israel Agricultural Institute.