1. Low phase-to-phase resistance...Each half is 32mOhms, so 16mOhms for the motor as a whole, so it can handle the current with low copper losses. eg 200A makes only 640W of heat in the copper, and 100A, a realistic highway cruising speed requirement, only 160W of heat. That's why phase-to-phase resistance is such a critical statistic for every motor, because it's the only way to predict the controller current limits you need to use.
2. High quality thin (.3mm) steel stator laminations. I know nothing about the grades of motor steel, but the factory literature always bragged about the use of high quality Japanese steel.
3. The low stator slot (24) and pole counts (10 pole pairs) compared to other hubmotors give it a low operating frequency, which combines with the high quality stator steel to result in quite low iron losses. eg at 100kph (62mph) the motor spins at 1089rpm with the tire size I use and iron losses are only 151W. Compare that to my V1 QS273 with it's .5mm lams and higher than a 205 slot and pole count that has no-load losses of over 800W at the same rpm. I'd have to run HubMonster at over 3000rpm to get to the same iron losses as my QS at 1000rpm. #2 and #3 combine in the motor stats as low no-load current for a given rpm. Voltage X no-load current is your iron losses.
4. Small air gap and curved magnets. If I remember correctly the magnetic gap is about .5mm . I think these close build tolerances fall out in the performance results as better torque per amp and less torque ripple (cogging torque), which I see in the form of these large hubmotors (15kg) coasting better than any hubmotor I've used.
As an example, at whatever wind resistance and grade cause the motor to draw 100A at 100kph, Miles' spreadsheet tells me the power output at the wheel is right at 6,000W and the motor is creating only 319W of heat (an amount of heat easily dissipated at that speed) for an overall efficiency of 94.9%. Then with the near doubling of power required for a 25% increase in speed to 125kph (78mph), HubMonster is putting out 11,656W on input of 12,270W , at slightly higher efficiency of 95%, and again it should be able to shed the 614W of heat generated, so continuous cruise at that speed is possible as long as your battery can last. Now at the well over 30kw peak input I run one of my HubMonsters at (260A on a 31s pack), I did ventilate the motor with exterior blades on the air output side, and in regular use it's high efficiency makes it my coolest running HubMonster that typically below 70°C with the highest I've seen being 100°C with hard acceleration out of every switchback really attacking a 20% continuous grade to visit the windmill farm on top of a mountain.