facebook chat sidebar- how did she make her picture dissapear, then reappear, then dissapear again

this is frustrating i have friends who can make themselves disappear from my chat sidebar, then reappear. I had a falling out with this one so i cant ask. But how do they do it. is it simply tuning chat on and off? Often my chat is off or its turned on for only 2 people. i get the feeling everyone can see me online anyway. I dont want to appear online to anyone if there is a way not to, especially with a time stamp of when my last message was sent.