Hi Lana89,
I've linked to a document below that gives a bit more information about how triggers work. Your USB 6009 device does have triggering capabilities, but it can only trigger off of a digital signal, so you wouldn't be able to use it on your analog task. Another important concept to understand is that triggers typically just tell a second task when to start, rather than triggering individual readings. The link below gives more information about triggers:
Tips and Techniques in Data Acquisition Triggering: http://www.ni.com/white-paper/4329/en/
That being said, rather than using a trigger, you could read in data from your first task and continously monitor it. When the signal changes, start the second task, take one reading, then unreserve the task. Your second task will have to be unreserved after each reading so that it can later be re-started to take the next reading. You can use the DAQmx Control Task VI to unreserve the task:
DAQmx Control Task: http://zone.ni.com/reference/en-XX/help/370469AA-01/lvdaqmx/mxtaskctrl/
Myriam D.
Applications Engineer
National Instruments
