  We investigate theoretically and experimentally the electron paramagnetic
resonance (EPR) spectra of bismuth doped silicon (Si:Bi) at intermediate
magnetic fields, $B \approx 0.05 -0.6$ T. We identify a previously unexplored
EPR regime of "cancellation-resonances"- where part of the hyperfine coupling
is resonant with the external field-induced splitting. We show this regime has
interesting and experimentally accessible consequences for spectroscopy and
quantum information applications. These include reduction of decoherence, fast
manipulation of the coupled nuclear-electron qubit system and line narrowing in
the multi-qubit case. We test our theoretical analysis by comparing with
experimental X-band (9.7 GHz) EPR spectra obtained in the intermediate field
regime.
