What does this equation mean?

Read the first line on its own and the trait looks stable: natural selection ( $\beta_t$ ) pulls the ornament back down every generation. But the second term, B\,$\beta_p$ , imports a push from the preference side through the genetic covariance the mating system itself has created. When B is large enough relative to the stabilising pull of $\beta_t$ , the two equations no longer settle to a fixed point; they feed each other, and $\bar t$ and $\bar p$ escalate together until some external limit — a wing that can no longer lift the tail, a display that finally costs more than it wins — brings the pair to a halt. Nothing in the equations names where that limit falls. That absence of an internal stopping rule is the entire content of “runaway”: once the covariance term dominates, the direction is set and the magnitude is set by biomechanics, not by the selective logic that started the process.