[ptryon]

Take the Haber process as an example:

N₂ + 3H₂  2NH₃

According to Le Chatalier's principle if you increase the pressure the equilibrium shifts in such a way to counteract the change, i.e. it shifts towards to the right where there are less moles of gas. Le Chatalier's principle also states that if you add more ammonia the equilibrium will shift to the left to reduce the amount of ammonia...

What happens if you increse the pressure by adding more ammonia? (keeping T and V constant)

[rabolisk]

It shifts to the left. The reason why an increase in pressure favors the side with less moles of gas is because the increase in general pressure (i.e. not due to adding any more reactant or product) is equivalent to decreasing the volume, or increasing the concentration for all gases. Mathematically, this results in Q decreasing for this reaction. If you just add ammonia and hold volume constant, then the concentration of hydrogen and nitrogen do not change, but the concentration of ammonia increases. Now Q > K, and the equilibrium shifts leftward.

[ptryon]

That makes sense. Thanks for your help. I forgot about Q as a predictor for the direction that an equilibrium will move- in my view it's much more useful than Chatalier's principle!! 
 

[Borek]

That makes sense. Thanks for your help. I forgot about Q as a predictor for the direction that an equilibrium will move- in my view it's much more useful than Chatalier's principle!! 

Please remember LeChatelier's principle is more of a rule of thumb than anything else, so its applicability is limited.