Phonetics and the IPA 8, Exercise 2: pots and pods

pots /pɒts/: the suffix is /s/, voiceless, because it follows the voiceless /t/.
pods /pɒdz/: the suffix is /z/, voiced, because it follows the voiced /d/.

Cause: the suffix takes its voicing from the preceding consonant.

Direction: the changed sound (the suffix) matches the sound BEFORE it, so
this is progressive (perseverative) assimilation. Regressive would be a
sound changing to match the one after it, as in handbag -> [ˈhæmbæɡ].