So when i try to use masking tool this happens
So i want to make my mask edges sharp like this how can i do that? ye i know im kinda new with this stuff 
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I think they simply did the drag rectangle starting in empty space mask.
Also, I’m learning a lot by watching the “Ask Zbrush” videos on Youtube and the Tutorials on their zbrush website.
When masking, the resolution of the model helps with the crispness of the mask. But in saying that, when masking you can blur the mask (click on model while holding down the Ctrl button), and then sharpen the mask by Ctrl-Alt and clicking on the model! Hope this helps.
Another way to get sharp edges is to duplicate the SubTool, then do your masking. Make the masked area into a new polygroup. Then get the Smooth Groups Brush in Lightbox-Brushes-Smooth and run that brush at a low intensity around the polygroup borders. Then when you extract it, you will have perfectly sharp edges.