With a classic American-style key, you operate it with the key set back on the operating table with your arm resting on the table for support. The first time you sit down with a large, "English-style" key like this Swedish Pump from Hi-Mound, you immediately feel that the knob is too high off the table for comfortable keying. In fact, it is too high and if you handle such a key like you would a J-38, the result will be immediate discomfort and fatigue.
A key like this requires a completely different technique, which, if you master it, you will find to be much more comfortable than the American-style keying you to which you are accustomed. First, the key is located near the front edge of the table, on the right or left side, depending on whether you are right or left handed. You grip the knob with your fingers (basically in any way that is comfortable) but your arm is hanging free in space - not supported by the table. This may sound very tiring, but take my word for the fact that it won't be! The position of the key and your sitting position should be such that your upper arm hangs essentially straight down from your shoulder and your forearm is essentially at right angles to the front of the table/desk. It is best if the height of the table/desk is such that the forearm is essentially level, but some latitude is possible.
The actual keying is accomplished by rythmic movements of the forearm with little or no flexing of the wrist. With an American-style key you typically set the gap very tight and then "micro-manage" the key with wrist and fingers. That's what gets very tiring and can lead to the dreaded "glass arm" - a painful condition caused by excessive tension and flexing of the wrist. With English-style keys the gap is much wider and you tend to bang away at the key with your forearm. You will immediately see where the phrase "pounding brass" came from - not from a whimpy J-38, I can assure you! It is almost all relatively low-impact large-motor work and, with modest practice, you can do it all day without getting tired or cramped up.
Bottom line? Were I forced to use a flat, American-style key, I would position it for English-style keying without a moment's hesitation.