The OpenSprinkler firmware sends out GPIO command to shift register every second (i.e. refreshes) to make sure the valve status remains in sync with the controller. If you want manual control through script, you will need to stop the firmware from running in the first place. Instructions are provided here:
https://opensprinkler.freshdesk.com/solution/articles/5000631599-installing-and-updating-the-unified-firmware#stopall