If you can provide a reference for where the term "key point diagram" is used that might help. Google finds none.
Let us know a little more & we might be able to point you in the right direction.
The best place to start in my opinion would be to check out Job Instruction (JI) Breakdown sheet. It is part of the Training Within Industry (TWI) program developed over 60 years ago. The basic concept has 3 components.
- What is the activity step
- What are the "Key Points" for the step
- Why are the key points important
It is typically used for OJT as talking points for the trainer, but I assume could be posted for shop aids.
More info can be found at TWI Institute
Here is a slide share that goes into more detail about the Job Instruction Breakdown.
Hope this helps.