If the +ve input goes, say, a little high then the output (of the op-amp) will try to slam hard against the positive rail (because the op-amp's gains is very high).
As soon as the output starts to move high, it is fed back to the -ve input (reduced by the potential divider of R7,8) and this "cancels" the above change that caused the output to move high.
This "action" makes +ve look like a "high impedance" - its the same thing as power steering: a slight displacement allows a powerful fource to "null it out", moving the wheels the while.
It is fun to work out; the input impedance of +ve... and then you will never have to depend on a "PDF" again!
Here is an example of how to do it for the simpler inverting op amp arrangement:
1) If -ve input changes by delta voltage "dV" (w.r.t. the +ve input). 2) Then the current at -ve will change by "dI". 3) And evidently dV / dI == input Z at -ve. ... a) Meanwhile the output moves -dV * A (A == open loop gain). b) And most of dI flows via the FB resistor (very little via -ve). c) And most of -dV * A volts is across the FB resistor (dv