Why is it optional?
If you are planning to fly your Evo extremely fast, for example with a 'hot' brushless motor and Lipoly cells, then you will need to add a carbon reinforcement to the Evo wing.
At extremely high speeds, the spar will prevent the airframe from deforming.
For Evo gliders and standard speed 400 aircraft this spar is not required.
However, if you are planning to upgrade your Evo powerplant in the future then it may be worth adding a spar now. Also, the spar is reusable and can be transferred into your next airframe.
Fitting the Spar
The spar is a carbon tube outside diameter 6mm, inside diameter 4mm and length 450mm. It weighs nominally 18g. This tube is available from Mugi.
It should be placed half way between the trailing edge of your Evo and the servos. You can fix the spar in place by using self-adhesive tape.