Variables aren't checked continuously while a flow is running.
So, for instance, if changing the value of a variable to 1 triggers a flow, then changing it to 0 after that flow is triggered will not interrupt the flow. Unless every step in that flow is configured to check the value of that variable using an "only if".
Because each Stringify account is given a small number of variables, one would rapidly run out of variables were this method to be used.
If you use the Stringify app to change the state of an object used in the flow (eg. turn light on/off, change color, change brightness), any flow that the object is used in will be interrupted. For example, if a flow uses a Hue bulb, and you change the status of the bulb using the Stringify app, then the flow will be interrupted. It will not be interrupted if you use the Hue app or any other Hue integration like Alexa/GH.