MA plot interpretation before and after normalization

Question

[Expe]

I have a RNA-seq dataset of tumor and control samples. I plotted two MA plots, before and after normalization (plotSmear with Bioconductor). I don't understand how to know it the results are ok just by looking at the shape. My professor explained that if the 'dot cloud' has a trumpet shape, it is good. But then in some exercise examples he shows MA plots with different shapes that are also ok. I would appreciate an explanation for 'dummies' on this as I am a newbie. I forgot to say that I have already read the wikipedia page about MA plots and some articles about RNA-seq with no luck. Thanks in advance.

[BioinfGuru]

It would be better to give an image of the plots in question.

If you can't provide that then please tell us what are your X and Y axis labels?

[Expe]

Sorry for my late reply. The X label is logCPM and the Y label logFC (tumor-control).