In this type of puzzle some spokes have to be drawn between the given hubs. A digit on such a hub tells how many spokes meet there and the spokes are not allowed to cross anywhere. In the end everything will be connected.
Small thingies are mounted on the hubs. When we click in the neighbourhood of such a thingy with the left mouse button, a spoke will appear that connects it with its counterpart on the opposite hub. A right click has another result, namely that two little transversely drawn stripes appear to indicate that a spoke will remain to be unused. Using these is not obligatory and they will be ignored when the puzzle is checked for correctness.
We will take a look at a small puzzle of this type and solve it. The hub in the upper left corner is a good starting point. It needs to be connected by three spokes, but since it is in a corner, there are only three neighbouring hubs. We can draw these three spokes immediately. In the same way, we can draw three spokes at the bottom left corner.
We now observe that the middle left hub, that needs two spokes, already has both of them. This means that its remaining connection points will not be used, so we cross them out. The same reasoning applies to the central hub. The middle right hub needs four spokes, and since
it cannot be connected to the central hub anymore we need to use all
its remaining connection points.
The puzzle is now almost completed, there are just two hubs left that
do not have the required number of spokes. Luckily they are next to each
other, so we add one more spoke between them to finish the puzzle.