Lighting truss is designed to be like a hollow box with most of the weight taken out, the chords are thin walled typically 2mm, it`s very strong in compression. As shown apex up, lights would be hung below and the forces compress the top chord.

But as enough recent stage collapses have shown, push it in the middle and lose most of its strength.

That`s my very basic understanding, which may not be entirely accurate, not a rigger nor do play one on T.V. ;-)