Need help understanding crc hash used to explain test train split in Chapter 2

Hi there, in the following code, I fail to understand how the hash function is working here and how would it make sure that the test set is randomly selected. Can you shed some light on this please? Thank you.

```python
def test_set_check(identifier,test_ratio):
return crc32(np.int64(identifier)) & 0xffffffff < test_ratio *2**32)
```