Here is something critical and missing in the excellent article about the supply tips, when you play as the defenser side, or if you prefer, when your are the one who CLOSE the turn, before the replacement/supply updating of the nex turn :
With this example, you can see how the supply of a unit works with his adjacent hex, and the presence of opponent units :
In this situation, there is no problem the unit is supplied...
Now look at here with a slight difference of the units positions :
the german unit is in a surrounded position, if you let it like this, the unit will finish to decrease until 1% supply and the materiel will slowly decrease also..There is an exception ofcourse if your unit is just located on a supply source.
Actually you can even cut the supply of any unit of group of unit which are on 1 hex, with only 2 of your unit arround the sides.
You know the tiny shitty unit which look useless with [1 1] as attack/defense, use it to encircle stronger opponent units...
Even if your little unit get blitzed on the following turn, you will still have prevented the supply update of 1 or more opponent units..
This is in my opinion the big advantage of the defense side, when you try to do this in being the attacker, then the defenser can still react in moving his units to avoid these situations before the turn updating.