Best way to repair cracked attic post?
That vertical member is a truss chord not a post. It’s in compression (as @SteveSh) has identified.
It’s buckling due to an excessive vertical load (and probably because of a small defect in the wood).
This phenomenon is identified in Euler’s Formula. While I don’t understand the formula, the concept we know it by is the “slenderness ratio”. That is to say, a short board can support more load than a long board before it buckles, given the same size, material, etc. Likewise, the member will buckle in the narrow direction.
The center chords are the longest member in the trusses AND are in compression. This particular member has a small defect near the middle of the span (top to bottom). It also happens to be at the hip in the roof, so it’s probably carrying a slightly larger load than the adjacent trusses. (We often see defects in chords that cause them to fail.)
I’d repair the chord by sandwiching it between similar size members, much like what daneb described.