There is only one substance that can unlock the B17 molecule and release the cyanide. That substance is an enzyme called "beta-glucosidase," which is called the "unlocking enzyme." When B17 comes in contact with this enzyme in the presence of water, not only is the cyanide released, but also the benzaldehyde, which is highly toxic by itself.
In fact, these two working together are at least a hundred times more poisonous than either of them separately; a phenomenon know in biochemistry as synergism. Fortunately, the "unlocking enzyme" is not found to any dangerous degree anywhere in the body "except at the cancer cell," where it always is present in great quantity, sometimes at levels in excess of one hundred times that of the surrounding normal cells.
The result is that the vitamin B17 is unlocked at the cancer cell, releases its poisons to the cancer cell and ONLY TO THE CANCER CELL.
Ref. WWC, pp103