  Parahydrogen (pH2) is a convenient and cost efficient source for magnetic
resonance signal enhancement. Transient interaction of pH2 with a metal organic
complex in a signal amplification by reversible exchange (SABRE) experiment
enabled more than 10% polarization for some 15N molecules. Here, we analyzed a
variant of SABRE, consisting of an outer magnetic field alternating between a
low field of ~1 \muT, where a polarization transfer takes place, and a higher
field >50 \muT (alt-SABRE). We found effects of both of these fields on
amplitude and the frequency of polarization transfer. Deviation of a lower
magnetic field from a "perfect" condition of level anti-crossing increases the
frequency of polarization transfer that can be exploited for polarization of
short-lived transient SABRE complexes i.e. some substrates. Moreover, the
coherences responsible for polarization transfer at a lower field persisted
during magnetic field variation and continued their spin evolution at higher
field with a frequency of 2.5 kHz at 54 \muT. The latter should be taken into
consideration for an efficient alt-SABRE.
