Duolingo did not invent streaks but they proved the model: a visible counter you do not want to lose is more motivating than any reward. The mechanism is loss aversion, and it is one of the most replicated findings in behavioural economics.
What most apps get wrong
They make breaking the streak free. A reset is a soft penalty — annoying, not painful. Cues adds a chicken-out counter that compounds over a rolling 7-day window. Skip three days and your daily quota doubles. The cost of avoidance becomes real.
Why this maps to confidence
Confidence is downstream of action. Action requires a forcing function. The streak is the forcing function. The chicken-out tax is the consequence. Combined, they remove the negotiation in your head about whether to approach today.
The catch
You have to log honestly. Cues uses a few signals to keep people honest, but ultimately the system rewards the man who would already do the work. It just makes it easier to keep going.