Here is what the Manual says about Interdiction:
"Interdiction is enemy activity which interferes with supply lines
and/or which makes it more difficult for units to move."
It goes on to explain that air & partisan interdiction can interfere with supply but doesn't explain the exact effect. For example, here is what is says about air interdiction in particular:
"Air Interdiction is the ability of aircraft to attack a road or junction to slow movement and/or supplies."
This would indicate it only slows supply but if it can slow supply in one hex & this effect is magnified by the interdiction affecting a number of contiguous road hexes then it might, in effect & obviously, block supply completely.
Will air or partisan interdiction slow or will it completely block supply delivery to an enemy unit? Or both? Or does it depend? On what factors, then, does it depend? How would I know?
In other words how would I know the effect it is having?
Now (here is the hard one) where exactly in the Manual does it explain the precise effect interdiction has on an enemy unit beyond what I have just quoted from the Manual?
ORIGINAL: Joe 98
As you know units have “bullets”
Some units have only 2 bullets.
When a unit moves, attacks, digs in or does most other actions, it uses up a bullet.
If you attack an enemy, this causes the enemy to use up a bullet.
So, if you can cut the enemy supply route with your zones of control, or with air interdiction or with artillery interdiction he is out of supply.
When you attack that enemy unit, the enemy unit uses up a bullet.
If a unit has only 2 bullets it takes only 2 turns to use them up.
If unit has used up all it’s bullets it “goes into the red” ( the background turns bright red) and it is easy to destroy.
From this we learn: if you are on the defence, you’re units that have only 2 bullets, should be positioned on roads so supplies flow easily.
If on the attack, use units with 4 bullets. Try to break through down roads and turn and cut supply to enemy units that are away from the roads.