The blackest of black magic. We use 3 drops of unicorn blood and sacrifice a bushel of fava beans in the creation of each IEM.
It's just good, clean engineering of a transducer and the acoustic volume in front and behind the transducer. And really good materials. We use polycarbonate for the housing a reason, rather than metal. it simply is an acoustically superior material. The fact it's light weight, doesn't feel cold OR hot to the touch, and is lower cost is all gravy.
We also make sure that our IEMs play nicely with cell phones - which guarantees excellent results with amps and DAPs. A nice and high 32 Ohm impedance and high sensitivity (as high as allowed per EN50332) make them work really well with cell phones. Portable means you shouldn't have to have an extra bit of kit to be satisfying. Yes, an external amp or DAP helps, but you can get very good results with just a phone (I listen to mine about 70% of the time straight off my Samsung Note 5 - Tidal, streaming or offline media).