determining whether there is any coupling between the pairs of bits that would be xor'd together

I read in that same text that in a good digest function, each bit of the input should affect approximately half of the bits of the output. (I think) the implication was that if this ideal is achieved, and the distribution of the affected bits for each bit of input is sufficiently unpredictable, then the digest is 'secure' meaning unpredictable. But the problem with many of the digests that have been shown to be weak, is that the ideal is not achieved, making certain patterns of outputs for given inputs discernable.

I read that to mean that there is coupling between the upper and lower halves of any digest. Good or bad?