Coach levels

[fizban]

I was wondering how the coach levels work. In other chess apps, it seems that lower levels tend to make more frequent obvious blunders. In pbchess, I noticed blatant blunders up to level 3. Level 4 looks like as if it had a ply depth of 2 or if blunders were more spaced in time, while higher levels might increase the ply depth/time. On which engine is based this coach? How is its strength limited to create the different levels?