Thats a great question. I have no idea how it works, but I would imagine that once you sign up for a plan and an account, you would manage it from their internet site.
EDIT: Looking at their login page, it seems you can manage your account online. So I would think you can start and stop your plan from that page.
https://www.clear.com/my_account/signin.php
Thanks! More digging, starting from that point, turned up this:
https://www.clear.com/support/faq
(click on the iSpot tab for details)
After a fast read through that FAQ, I think they seem pretty focused on making this a full time service, not a true "as you need it" or Pay As You Go service. Stopping and starting, only using it a few times a year--not really what they intend to have you do. It's probably doable, they're just not interested at this time in making it relatively painless. I'll read it a little more thoroughly, but that combined with the other limitations...eh, not as impressed as I was at first glance. It'll be useful for some people, but I think many are going to be better off with other options.
Edit: on a closer look--to be fair, it can be done, and they don't limit how frequently you "suspend" service and resume it. The issue for me is their suspension period is 60 days, non extendable. There are going to be periods where I just don't need it enough to pay the $25 monthly fee to reactivate. I need to think this through.
Edit 2: well, after quite a bit of thought, it's still the coverage that's the deal breaker for me. I just isn't useful to me for travel, because there are so few areas that they cover in 4G, and this unit isn't compatible with their nationwide 3G coverage. Realistically, in my case, it just wouldn't get used--I have unlimited 3G on the iPhone that's reasonably fast, highspeed cable at home, wifi nearly everywhere I go, and this would barely cover a fraction of the travel I'd need it for for the remainder of the year. I honestly can't quite see how they'll make this a success without adding the more widespread 3G coverage.