The power valve opens the second the throttle is on the floor(as soon as it reaches the rating on the valve), it only responds to manifold vacuum. Most setups will get way below the pv's opening point even at idle and full throttle. The pump shot covers the time period it takes for the fuel to accelerate through the carb's passages and out into the airstream. Since the fuel is denser than the air, any given volume of fuel has more inertia to overcome than a given volume of air, and moves more slowly. Pump tuning can be a REAL pia too, since you have to balance total volume with delivery rate and delivery volume vs throttle position. Trying to get those little hiccups and burbles out can be very, VERY frustrating. As for the power valve, it'll be most noticable if it's wrong at part throttle near the valve's rated vacuum. If it's too small then you may get a part throttle lean flat spot, or too big and you may get a part throttle bog from too much gas.