If you are in a flex pool for a specific client, you will see a lightning bolt (⚡) in the shift window. You can claim this shift immediately. Congratulations! You can get started. If you are not in that flex pool, you will not see a lightning bolt in the shift window. You can then only "register" for the shift in question. Now all you have to do is wait until you are selected!
Click here to find information about the flex pools.