The analysis of the radical pair sublevel populations is presented for the combined action of the Larmor frequencies differences, hyperfine interactions, and the microwave fields. Some remarkable features of transitions from the singlet state to the triplet sublevels T0 and T+ (or T-) under the microwave pumping have been found. This knowledge is very instructive to get insight into the spin-locking effect in a recombination of spin-correlated radical pairs, and it also shows how a radical pair recombination can be controlled by the CW and pulse MW field.