When you build a workflow using some of the PowerShell tasks you may come across a need for the special variable "$_". This variable represents the current pipeline object in PowerShell and is used in workflow tasks to refer to the passed in 'Input' object. One case where this comes up often is with the "Where-Object" task. When using this object your "Filter Script" property will typically contain something that looks like this:

$_.Status -eq "Running"

The above means to return all items from the Input object where the Status field is "Running"

I found this site that does a great job of explaining the special variables in PowerShell if you want to explore further:

http://www.computerperformance.co.uk/powershell/powershell_variables.htm

That site has a lot of tutorial pages on PowerShell covering the escape character I talked about in my previous post as well as a good summary of PowerShell syntax. I find myself on that site often from searching for PowerShell topics and often find answers there.