Well if it says interview only, they mean interview only and that there's no other assessment such as college admissions tests or TSA etc.
You will probably have at 2 interviews - One with the director of Studies for Engineering at Christ's and one with a General Tutor of the college (or at least that was the case last year).
Christ's are very proud of their ability to hand pick who they believe are the best students through interview.The tests may or may not go to your advantage. If you're someone who gets very nervous speaking to someone or aren't entriely confident about your ability to speak about your subject, then a solid score on an admissions test can really boost your chances. Conversely, if the admissions test isnt your strong point, then it's probably a good thing that they dont have any admissions test.
You can probably expect the content of the interviews to be similar to what most other colleges do - one general interview and one subjects based interview (on mathematics and physics mainly)
Unfortunately, having not been an engineering applicant myself, I can't tell you more, I'm sorry!
Note: Last year, Engineering students at Christ's were expected to do the TSA. I don't know if their policy has changed since then so I advise that if you're seriously thinking about Christ's to email the Admissions Tutor and ask.
My interview experience with Christ's was very positive (although I didn't get interviewed for Engineering, I got interviewed for NatSci)
The admissions tutor was always happy to answer any questions about the admissions process, so if you have any worries, you could always email her and ask her - she's a lovely lady