A highball has two ingredients and no technique to hide behind. What separates a good one from a dull one is almost entirely how much carbonation reaches the drinker.
Why the format exposes everything
A cocktail with citrus, sugar and bitters has several components that can compensate for one another. A highball has a spirit, a mixer and ice.
With nothing to balance, every fault is visible. Warm mixer, wet ice or a heavy pour all show up directly in the glass.
The corollary is that small improvements are equally visible, which is why bars that take the format seriously can charge for it.
What actually removes the fizz
Dissolved carbon dioxide leaves a liquid whenever it meets a nucleation site or a rise in temperature, and a poured drink offers both in quantity.
Ice that has been sitting in a well is already melting, and its wet, pitted surface releases gas far faster than a hard, cold cube.
Stirring after the mixer goes in is the single largest loss. One gentle lift is enough, because the spirit and mixer combine largely on their own.
Why temperature is the controlling variable
Cold liquid holds gas; warm liquid releases it. A mixer stored at room temperature has lost a meaningful share of its carbonation before the glass is even full.
Bars serious about the drink refrigerate mixers, freeze the glass and keep the spirit itself cold, so that nothing in the build is working against the gas.
This is the reason Japanese bars became known for the format, and the reason American bars adopting it invested in ice and refrigeration first.
How ice shape changes the result
A tall glass filled with large clear ice has less surface area and less trapped air than the same volume of small machine cubes.
Less surface means slower melting, slower dilution and fewer sites for gas to escape from, all of which extend the life of the drink.
Filling the glass completely helps rather than hurts. A full glass keeps the ice from moving and warms more slowly than a half-filled one.
Why the pour order is not arbitrary
Building the spirit over ice first, then adding chilled mixer down the side of the glass, minimizes turbulence at the point where gas is most likely to escape.
Pouring the mixer first and dropping spirit through it does the opposite, agitating the carbonated liquid at the moment it is least stable.
None of these steps takes extra time once they are habit, which is why the difference between a careless highball and a careful one costs the bar almost nothing.