  Nitrogen is produced by CNO-cycling in massive stars, and can be ejected in
significant amounts in supernova explosions. While in H-rich SNe, its
[\ion{N}{II}] 6548, 6583 emission becomes obscured by strong H$\alpha$, in
explosions of He stars, this nitrogen emission becomes more visible. We here
explore the formation of this line, using the \texttt{SUMO} code to compute
spectra for a grid of 1D models with parameterized mixing informed from new 2D
simulations. Because the mass fraction of nitrogen in the ejecta decreases with
larger He core masses, as more of the He/N zone gets processed by shell helium
burning and is lost to winds, the [\ion{N}{II}] luminosity relative to the
overall optical flux probes the He core mass. By comparing to large samples of
data, we find that low-mass He cores ($M_{\rm preSN}\lesssim\ 3\ M_\odot$) are
exclusively associated with Type IIb SNe, with the exception of Type Ib SN
2007Y. Seeing no strong nitrogen emission in other Type Ib SNe, the implication
is either an origin from low-mass stars with the He/N layer (but not the He/C)
layer peeled away, or from higher-mass He cores. We also see no clear nitrogen
emission in Type Ic SNe. We discuss the diagnostic potential of this new line
metric, and also dependencies on mass-loss-rate and metallicity.
