On top of any thread, there's a link/button to switch that option on and off.
Default is off, that means all posts are visible, just like normal.
OP: xxxxxx
user01: jkljjklll
user02: jlkjskladfjkl
OP: yyyy
user03: kldsjfkljasd
OP: zzzzz
user04: jkldsjaf
When the switch is on, i.e. only...