IQbuds SH Reviews, page-27

Yes, I was surprised that the app user guide is not available on the Nuheara website via a direct link.  I'm not an iOS user, so I visited the App Store on my PC and found the Support link in the app description .. http://www.nuheara.com/nuheara-iqbuds-support/

By the look of it, the EQ adjustment acts on either an unspecified "low" or a "high" frequency band and as observed above, doesn't allow you to choose specific frequencies to boost or cancel.  That said, with the experience that the Nuheara team have in the audio field I imagine there is a lot of science behind the frequency band that is used for the low and high EQ adjustments - I imagine it probably functions like a parametric EQ that affects the EQ curve more broadly rather than a "notch" style EQ control.