The main purpose of honking is to transfer that part of your body weight that was on the saddle, onto the active crank, thus increasing the downward pressure on that crank with little perceived increase in effort. Therefore it makes sense to stay as upright as possible, putting your weight in line with the downward thrust of your legs.
However this is a very inefficient action, because it exaggerates the 'bottom dead centre' of each pedal stroke. If you value your circular pedalling, you counter this by getting your arms involved, pulling on the bars, which for best effect involves crouching forward to some degree. This effort can't be maintained for long periods - whereas honking upright can, on a long steady hill for example.
When I was a younger and stronger rider, I used to snap cranks regularly whilst honking - not on steep hills, but while using the big ring on a slight uphill drag. After the third time I learned to moderate my technique.