Toggling start and stop will automatically start and stop the lightbar's movement on your client's screen. Likewise, changes to the speed and width of the light will automatically be reflected in your client's browser.
You may create as many session URLs as you would like, even as many as one per client if you prefer. You will only be able to control one session URL at a time.
To get a feel for what your client will experience, open the session URL in a private tab and play with the lightbar settings in this tab.
The control over the lightbar in the session URL is locked to only your browser. If you use a new browser or a different computer, you will be given a new session URL. Remember, they're free, and you can generate as many as you'd like, so just be sure to share the one you're currently in control of with your client.
If you still need help, please email emdr_lightbar (at) fastmail.com for assistance