The stoker is never at fault.
Communicate. Many captains call out shifts. I don't, but my stoker is pretty used to my shifting habits and I aim to not do anything disruptive to our pedaling efforts. Agree that bumps should be called out. At stoplights, whoever notices yellow for the cross-street calls it out so we can be ready to go when we get the green.
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Bacchetta Giro A20, RANS V-Rex, RANS Screamer