What are the vertical pieces of metal on the Slingsby Swallow wing for?
You're correct -- they're speedbrakes, often referred to as spoilers, and are retracted when striving for efficient flight, but are generally extended in the landing pattern so that you have correction available if you start to get slightly low.  Also frequently extended when the glider is left on the ground to reduce the effect of gusty winds &amp; the likelihood that they would lift a wing.