Suppose that \(P_1\sim^\RR P_2\). Then there exists \(F\subset Q\times \RR\) with \(F_{q_i}=P_i\).
Take \(R=F\), and define \(\phi:=\pi^*\psi\). We see that \((\pi_Q)_* (\pi^*(\psi^*\psi)\cdot F)=P_1-P_2\).
NOw suppose instead that \(P_1\sim^b P_2\). Then there exists \(R\) and \(\phi\in \mathcal K_R\) such that \(f_*(\mathrm{Div}(\phi))=P_1-P_2\).
Tke \(F:=(f\times \id)_* (\text{graph}(\phi))\). Cruicially, the function \(\phi\) is bounded so that the intersection with a sufficiently large slice is empty