Research what happens in this under constrained

[motemotech]

What's the purpose of this research?

I would like to know what is happening in here

https://github.com/zkCert/halo2-rsa/blob/cec7f8a6092cf1512107680598b88d44485579d8/src/big_uint/chip.rs#L252-L255C23

Concrete milestone by steps

• [ ] Understand what is happening
• [ ] Report in documents and submit issue if we need to change somethign

Describe references if exists

No response