Here is more clean english.
First, make a mask, preferably Kokoru. Now, wait until combat is up and use stealth. Now, right click the monster mask. You will see that the stealth stat still appears next to the mask icon. Wait until you can use stealth again, and just mash that until you are stealth again (you will clearly see that you are transparent). While under this state, you do damage of your regular weapon, with rate of fire and range of your regular weapon, and you can also use skills. However, if you do use a skill while under monster mask state, you will be stuck immoble. To remove immobility, doubleclick a nearby monster and you will be moble after it is dead.
Why you should care...
Stealth mode with the speed of a kokoru, while you can recast your buffs. Pretty useful for thieving.