P2WPKH program is identified by an output containing 0{20-byte-public-key-hash}. The spending transaction witness must contain the proper key which hashes to the 20 byte hash specified in the output. This is different than the regular scriptPubKey format used so far (1976a914{pkh}88ac is not necessary anymore).

But, when we want to hash the P2WPKH transaction for signing, we suddenly prefix the 20 byte pkh from output with 19761914 and suffix it with 88ac. Why do we do this? Why not just use the 20 byte pkh? If we don't use the old srcriptPubKey format for redeeming transactions why do we use it in hashing?