Henry, WD5JFR wrote:
"Shorts can`t dissipate power, so how does a stub work?"

Henry also wrote:
"I know that a shorted 1/4 wave stub exhibits a very high impedance. But
for the 2nd harmonic it`s a 1/2 wave stub and exhibits a very low
impedance or a short."

Henry is correct.

Connect a resistance directly to a transmitter. How much energy is
absorbed by the resistance? Ohm`s law is the first approximation.
Current is directly proportional to the applied voltage if the
transmitter`s internal reistance is negligible.

The shorted 1/4-wave stub exhibits an open circuit at its mouth and
accepts only enough current to supply its losss which are none in the
perfect stub. So, it takes no power from the transmitter afer its
circulating current is etablished.
At 2X the 1/4-wave freqency, the stub is 1/2 wavelengh and does not
transform the short at one end to an open circuit at its other end.
Instead, the 1/2-wave directly presents the short circuit at its far
end.

How much curent can the transmitter supply to a short circuit? It
depends on the internal impedance of the transmitter.

"Transmission Lines, Antennas, and Wave Guides" by King, Mimno, and Wing
explains how "Even Harmonics" are suppressed by the 1/4-wave
short-circuited stub on page 29. The gist is that the stub is imperfect
and its resistance saps harmonic energy which is allowed into the stub
by its low impedance.

Best regards, Richard Harrison, KB5WZI