Feynman Diagram 2

Regulator insertion $\partial_t R_{k,ab}(p_1,p_2)$, drawn as a circled cross, sitting on the propagator between external fields $\varphi_a$ and $\varphi_b$. In the functional renormalization group this vertex is what keeps the loop integrals finite: its scale derivative is peaked on the momentum shell currently being integrated out, so only fluctuations with $|p| \approx k$ feed into the flow.