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Thus the device decreases it conduction equal to its transconductance, if the idle current was high enough to absorb that amount that is; or it ceases to connection altogether, if the idle current is not high enough. And a negative going pulse will provoke an increase in conduction equal to the transconductance of the device. This is classic follower action in a nutshell.
On the other hand, when the top device is output referenced, the positive going pulse is relayed in its entirety to the input of the top device. In other words, the cathode or source or emitter may have been bumped up 1 volt, but then so has the grid or gate or base. And as it is the voltage relationship between grid and cathode or gate and source or base and emitter that controls the flow of current through the output device, the output device effectively does not see the pulse. This explains why the output impedance is so high. (The triode has an advantage here, as its rp will buck the pulse, which will offer some output resistance.)
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The answer is the same place the gain came from when the load attached to the plate or drain or collector: the transconductance of the device gives rise to a change in current flow, which when this flow also travels through the load, creates a voltage across the load's resistance or impedance. Thus, the rough equation for gain is gm against the load: gain = gmRL. The top device sees the same magnitude of input signal from its input to its output s the bottom device sees. The only difference is that the bottom device's voltage reference is ground and the top device's reference is the output of the amplifier. If the load is a dead short to ground, then both top and bottom devices become ground referenced and the drive voltage relationships become obvious.
Admittedly, when the load impedance is some value greater than zero, the drive relations become more difficult to see. Still, as long as we remember that the reference has moved from ground to the output, we stand a better chance of mentally unraveling the drive voltage relationships. Here is a different example of a change in reference, in point of view.
A man stands in a large basket-like structure. In front of him his a small vertical lever that is labeled "Up" at the top and "Down" at the bottom. Finding the lever at the mid-position, he pulls the lever completely up and then completely down. As he stares at it, the lever has only traveled half a foot from center to either extreme. But to the onlookers on the street, who see a man standing in the container at the end of fire engine's crane-ladder, the lever (along with the man) actually travels 20 ft from one extreme to the other. Who is right? What is your reference? If it is that of the man in the basket, then the lever only moved half a foot from its center position. If it is that of the onlookers on the street, then the lever actually moved 10 feet up (plus the half a foot) and then 10 feet (plus the half a foot) down from its center position.
After that long recapitulation of how this circuit works, let's turn to how the power supply noise leaks into output in this circuit.
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