Sternum chin-ups (where you lean back on the way up and touch your sternum to the bar) are great, but they can be problematic for some lifters. Sternum plank chin-ups may be a better option. They smoke the upper back in a similar fashion, but they’re a lot more user-friendly.
For one, they’re safer because you aren’t asking your body to bend backwards excessively. They’re also easier, so more people will be able to do them. Granted, they still aren’t easy by any means and should definitely be considered a more advanced variation, but once you can comfortably do 10-plus regular chin-ups with good form, they should be well within your grasp.
Lean way back as you pull yourself up, just as you would with a regular sternum chin-up, but rather than arch your back excessively, keep your entire body as straight as a board the whole time. Pull until your lower chest touches the bar and lower back down under control. The more you lean back, the harder it is.
As an added bonus, they double as one hell of a core exercise if you do them correctly.