you can't dislodge yourself. if you support another country's unit attack on your unit, and it would dislodge it, it doesn't work either.

djbent stop answering right when i m about to answer

What do you mean it doesn't work either - do you mean supporting another countries unit onto your own doesn't work? Or it would dislodge it? Thanks for the response :)

it would not dislodge it.

Supporting a foreign unit into a space occupied by your own unit can't help to dislodge it, but the support is good for a stand-off.

One example shows both principles:

Germany
A Silesia to Galicia

Russia
A Ukraine supports Silesia to Galicia
A Rumania supports Silesia to Galicia
A Galicia to Vienna

Austria
A Vienna to Galicia
A Budapest supports Vienna to Galicia

Result: nobody moves.

There are two attacks on Galicia, an Austrian attack of strength 2, a German attack of strength 3. Ordinarily the strength 3 German army would make it into Galicia. However there is a Russian unit that failed to make it out of Galicia, and two of the supports for the German attack are Russian. The Russian units give just enough support that the Austrian attack is stood off, but not enough that Galicia is dislodged.

Thanks for the clarification, that's really helpful

This can't be used to stop an attack however. If you have two units AB, and another nation has two units XY.
A hold
B support X->A
X move A
Y support X->A
The support given by B will be dropped, but the unit will still have support from Y and thus dislodge A

Also note, that if the area is NOT occupied, you CAN support yourself to win the battle over yourself...
A Por, A Mars, F MAO
A Por-Spa
A Mars-Spa
F MAO support A Por-Spa

Even if you own all 3 units, A Por-Spa WILL win and go to Spa over the Mars army.

This is used by the clever enemy when he realizes you are just going to bounce an area, but forces you to take it which opens an SC...
Similar example:
You are france with A Por, A Mars
Italy is your enemy with F MAO, A Pie
You want to keep him from taking Por, Mars, and Spa, so you order a bounce:
A Por-Spa, A Mars-Spa
The clever italy, though, orders F MAO support A Mars-Spa, then A Pie-Mars
So you're bounce becomes Mars taking spa, opening up Pie to take Mars.

It's a rare move, but I've seen it used a few times...