I gave a sketch here: Nick Nicholas’ answer to What are the pros and cons of the Erasmian pronunciation?
Erasmian is an early reconstruction of Ancient Greek. Compared to our current reconstruction, it’s not as distant from modern Western European languages, and it gets modified for ease of teaching in different countries.
Erasmian as taught tends to make the following concessions. (Happy to be corrected.)
- Usually stress rather than pitch accent
- Often fricatives rather than aspirated stops
- Nativisation of diphthongs
- Some distortions in the German version
- Even more distortions in the French version