protected void decrease() {
        value -= step;
        if (constrained && value < min) {
            if (wrapValues) {
                value = max;
            } else {
                value = min;
                timer.cancel();
            }
        }
        ValueChangeEvent.fire(this, value);
    }
Decreases the current value of the spinner by subtracting current step
stroom-core-client-widget/src/main/java/stroom/widget/valuespinner/client/Spinner.java