The "nonce" in a block of bitcoin made of a 32-bit (4-byte) field and its value is set in such a way that the hash of the block contains a run of preceding zeros. The remaining of the fields are left unchanged, as they have a defined meaning. A change into the data of block (such as the nonce) will make the hash of the block completely different. As it is considered to be infeasible to predict which combination of bits will result in the correct hash, many variety of nonce values are tried, and the hash is computed again for every value till a hash with the required number of zero bits is derived. The number of zero bits in demand is set by the difficulty. The hash as result has to be a value less than the current difficulty and hence would have to have a fix number of preceding zero bits to be less than earlier. The presentation of the nonce block of correct value constitutes POW, due the reason that the recurring calculation needs somewhat time with resources.
