Downfield (to the left) is in the direction of low magnetic field strength. Substitutions, such as an oxygen atom in an aldehyde withdraws electrons, thereby decreasing the shielding of the protons by electrons. In this case you need a lower magnetic field (downfield) to make the protons absorb EM energy from a constant strength magnetic field.
However substitutents, such as a methyl group donates electrons, which shields the protons, and in this case you need a stronger magnetic field, upfield, to the right.
