it's described in this study:
https://www.biorxiv.org/content/10.1....426186v1.full
it can be downloaded here:
https://github.com/cissy123/YHP-Y-Haplogroup-Predictor-
It's based on machine learning. According to the study, you can insert custom STR data for any haplogroup/subclade, and the program will "learn" to predict those subclades.
However, the instructions are very vague, and I can't get anything to work, even the basic prediction.
If anybody figures it out, please share how you did it.