Review sessions ("Strenght your skills") are really good, but I should think they would be even better if in the same session there were questions from various previous lesson. (At present, the questions are from just one or two lessons at a time, usually lessons in the same level at the tree)
I agree with mateusdl. The Review session would be much more effective if the weakened skills were selected randomly. The current top-down order makes the Review way too predictable.
The Review is for skills that have weakened, with the order of skills strengthened going from the top of the tree to the bottom.