Tracksetta is a brand of rigid metal templates designed to allow you to make sure that you are setting your flexi-track up in the correct manner to ensure that there are no kinks or sudden changes in the curvature that would cause a derailment of your train. By combining the different radii, it smooths out transitions and they  make laying track a lot simpler. The slots are positioned to allow nails to be put into the track and then slide the tracketta piece to do the next piece of rail. Investing time at this stage makes running your trains later, that much more enjoyable as they are running on extremely smooth and well laid track