It’s just a loop which continues running until the condition becomes false. More on reddit.com

After reading what you're asking, I'm scared to see your code. "Too many shift registers..." - check out clusters, they help you group data together. That way you can have a lot of different data on 1 shift register http://www.ni.com/video/3233/en/ What I think you're asking is to have multiple loops all pump data into one waveform chart. I'm assuming you mean a multiplot chart, and each loop will be writing into a different plot? Or is each loop writing to a separate chart? What is your supposed to do at a high level? It sounds like you're asking for help on a small section of code, when the real problem is the large section of the code. More on reddit.com

this happens because the default thread configuration prevents you from interrupting or duping another instance of a running hotkey, which also prevents you from assigning the variable. limiting them to a single thread avoids stacking hotkeys on themselves through auto-repeat and runaway loops, and other poorly implemented logic especially while hooks are not active

so you have 2 basic options, either increase thread limit through #MaxThreadsPerHotkey, or switch to timers. I prefer the latter, since timers are asynchronous and free up your keys to make any assignments or enable/disable other timers. they also have default intervals so your routines are not running balls to the wall, without any downtime your loop is really just spamming the target app with way more input than it can handle. example,

clickmonster:
	Send {LButton Down}
	Sleep, % Random(10,29)
	Send {LButton Up}
	Sleep, % Random(50,85)
return

; toggles clickmonster on/off @ 250ms interval
$=::settimer clickmonster, % (clickmonster := !clickmonster) ? "on" : "off"

you generally want to avoid running persistent loops directly under hotkeys, my personal rule of thumb is if the break/return condition happens well after releasing the button that initiated this loop, put it in a timer or subroutine

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First. See the FAQ for how to format code. Second, what are you trying to do here? It looks to me like you are constantly resetting the watch once you have waited for 100 milliseconds? No mechanism in your code will ever exit the while loop. How did you expect it to end? More on reddit.com