When a yo-yo that has a string wrapped tightly around its axle reaches its full extension, it automatically bounces back upwards, the string re-winding in the process. What causes it to do that? I haven't been able to find a satisfactory explanation anywhere.
The string is terminated by a small loop around the axle. The loop is just a bit larger than the diameter of the axle so that, when the yoyo is fully unwound and spinning rapidly, the yoyo can continue spinning with its axle sliding on the lower part of the small loop. The tension due to the weight of the yoyo stops the string being swept around the axle. To make the yoyo come back you must slighly drop you hand so the tension in the string falls to zero and the spining yoyo drags the string round the axles, catching it, and starts winding it up --- so trading rotational KE for gravitational PE. There is an art to this manoeuvre! It takes some practice.