Centerline has 75 page document that describes how these test work and what they are looking for. They will send it to you when you sign up for their interview prep. the really cool thing is they said if you don't pass they will refund your money.

There is some stuff on the internet that also describes the tests.
there is nothing you can study. they are basically trying to characterize your personality traits by what a bunch of psychologist think are everyone's traits. Even if you tried to tailor your answers to show certain traits, you don't know what United is looking for.

I have taken this type of test twice in the past for different airlines. Both times i passed, but it still worries me because I know some great pilots that have not.

I got the email that I have been selected to start the process yesterday. It said they would send the link to the Hogan soon, but I have not received it yet. How long did it take to get the Hogan link after you got the email from UAL?