[WM6]

Of course all is about magnetic influence of changeable magnetic field.

But, nulling existing signal pointed ferrite antenna at its source, is not due ferrite, it is due electronic circuit design after ferrite antenna. It can be unintentionally bad (saturation of first stage at some input signal level) or intentionally good, depend of our expected achievement.

Nulling of existing signal can be made due receiving reflected signal in counter phase too (as pa example tin hut somewhere behind you).