The passing six-four (P 6/4) typically connects a root-position chord with its first inversion counterpart (Ex. 7.5 & 7.6). The two essential passing motion moving in contrary motion (3-2-1 against 1-2-3)creates a voice-exchange. The idea of an exchange may be understood as two voice parts swopping notes. So, in Ex. 7.6a, the soprano Eb goes to the bass whilst the bass C is given to the soprano.
Ex. 8.5 - Can you locate the P 4/3 (and two C 6/4)?
Variants of P6/4s are possible. For example, a passing V 6/4 can be enriched with a chordal seventh (V 4/3) or substituted with a viio6 or viio6/5. See Ex. 9.1-9.3.
Ex. 9.8 sums up the three common versions.