Signal Flow Diagram for a biquadratic section
The relation between the circuit diagram and the signal flow diagram will become clearer when you have the corresponding 2nd-order transfer function H(s).Note that you can define three different functions (you have three output terminals) - but all with the same denominator (V2 and V3 are the lowpass outputs, V1 gives a bandpass function).

In this case, you will see that the resitor R6 controls the Q-value of the second-order filter function. This Q-value determines the lowpass characteristic (Butterworth, Chebyshev,...) and the bandpass selectivity, respectively. The corresponding formula for Q can be derived from the transfer function.

More than that, the first block is an inverting first-order lowpass (identical to an integrator stage with damping) which can add two different signals (feedback and input). Hence, you can split this block into two separate units: A summing block and a damped integrator (integrator with additional resistive feedback).  

The factor K is simply the DC gain of the lowpass filter identical to R4/P2.

EDIT:
I suggest to find the transfer functions for both - the circuit diagram as well as for the signal flow diagram. Then you can compare the various parts of the denominator and find the relations between both diagrams. However, it is important, that both functions are written in the so-called "normal form", which means: Nominator N(s)=(1+s*A+s²*B).