Where Is A Good Resource For Finding Out How Gearboxes Work On Diesel Trains?

I have been playing the new Blackpool DLC and I can't help but wonder how the gearbox works on the diesel trains. It literally sounds like a clutch engaging then the gear change happening, but obviously all automatic.

I have had a look around YouTube but nothing has really told me anything as a lot of videos focus on the whole train system rather than just what I want to learn. Thanks.

 

That should give you an understanding (3:40 on):

 

I'll add to this, while meant for older DMUs, I found these videos from the 50's on YouTube awhile back and they are fascinating...

Part 1


Part 2


Part 3


Part 4

 

That's great. Thank you both