The book says that when markets are in backwardation, a positive return is earned from a buy and hold strategy. Backwardation is when Futures price < spot price. Roll return = change in futures price - change in spot. This all makes sense because if the futures price is less than the spot then it rises to the spot price. so say:
Futures price at time t = 2 and spot = 3; then at time t+1 the Futures price would = 3 and roll return would be 1. this indicates a positive roll yield while in backwardation.
next example: If (RiskFree-convenience+storage )))))))=0 then the Futures price is = to the spot. no matter what the spot does (go up, down) the futures price will converge and there will be no roll yield (correct?). so say there is a problem with the orange harvest this year and the spot price goes from 32 to 104, because the futures price will converge to the 104 final spot, the roll yield would be (104-32) - (104-32) = 0.
It doesn’t matter what the expectations are for the spot at time t, if the futures price = the spot at the beginning of the contract then there is no roll yield.