You need to draw a pentagram in the dirt using the knife, forming a continuous line of blue light.
Here's what the walkthrough says:
Use the knife on the blue light. Click-hold the knife on the blue light to form an unbroken line that will not break. Form a pentagram. It is best to have a steady speed. Remember where the corners are located and where to change direction.
If error occurs, it will start from another point of light. The longer the line is unbroken, the brighter it becomes.
And a few extra pointers:
The light will disappear if you move the knife away from the course that the blue light is on, or if you hesitate and stop the knife altogether, even if it is only for a second or two. The puzzle then resets to a different point. The trick here is to try over and gradually learn the shape of the pentagram and where the turns are located. That way you can anticipate the lines and turns as you follow the light with the knife.
