Diamagnetic anisotropy is an effect caused by the pi electrons inducing their own magnetic field when placed under an external magnetic field.
The pi electrons aren't held as tightly together and so behave differently to sigma electrons.
Figure 1: Diagram showing diamagnetic anisotropy in double bonds and aromatic systems
One region of the induced field opposes the applied field which is equivalent to the effect of shielding. The other aligns with the direction of the field which has an equivalent effect to deshielding.
This is why in carbon to carbon double bonds we will see varying chemical shifts for both carbons.