I don't think that will work the way you want if you don't set up the trigger the way I showed you. That is how the info within the URL will be used by your piston. That's what I've been trying to tell you!!!
Update: I just tried it using the execution URL and IT DOESN'T WORK!!! You have to set up the trigger by the virtual device IFTTT and build the URL the way I showed you. If you use the execution URL, it will just execute the piston using the previous settings of the variable instead of updating to the new event description.