Hello Steiner,
Thank you for your question. The new design sensor with 3 prongs can replace any of the other detachable sensors. We added this 3 prong sensor so it can comb through thick hair easily. It is recommended to be used on the Pz electrode position (the electrode that goes on the back of the head). the sensor replacement pack fit with the kickstarter Insight headset.
The sensor that falls behind your ear are the reference sensors. We do not replace this.
If you have other questions, please let me know.