I once needed to jump-start a small marine diesel, many miles from land...
There was a small lever that cuts compression. You have to get it spinning really fast before restoring compression! It's definitely a lot of work!
EDIT - Here is a cheap modern small marine diesel [1]. The operation manual suggests that you don't have to do anything to get it spinning quickly, you just have to crank it 10 times, put away the crank handle, and then flip the compression switch. That's progress!
[1] https://www.yanmar.com/marine/product/engines/1gm10-marine-d...
Cranks and decompression levers are gone for at least 30-40 years now tho.
They're my kryptonite, but I accept it's mostly my ignorance.