I'm a bit slow, I know and could you please elaborate a little?
"Therefore, you must save your slow action in the round to be able to make an overwatch attack" refers to the attack, not to assume the position, does it not?
I was wondering whether a combatant can assume the overwatch position, not to make an overwatch attack.
Although, if a combatant isn't eligible to make an attack, then he might not be able to assume the position? (However, a combatant with a scope can assume the position without being able to make the attack, right?).
You first have to assume overwatch position. This is normally a fast action, but is a slow action if you have a telescopic sight. This allows you to aim at a selected hex and at anytime after that, you can fire your weapon at any one target in that hex. This count as an aimed ranged combat attack and is a slow action. In order for you to do this you need to have a slow action available.
As already been mention above here is how it goes.
Round 1
You spend your fast action to assume overwatch position. You select a hex and that's it. You save your slow action so you can use that for the overwatch fire action. At any time after this until the start of your next turn, you can execute this overwatch fire (and thus spend your saved slow action).
Round 2
You don't need to assume overwatch position again (unless you want to switch hex or something), but you can just keep being in overwatch at the same hex. Again you save your slow action so you can use it to shoot something in the hex.
Overwatch is broken if you do anything other than overwatch fire or if you are attacked in melee or if you suffer damage or if you become suppressed.
If you have a telescopic sight, then it is a slow action to assume overwatch position, which means that you don't have any slow action left for the overwatch fire, in this case you have to wait until your next round, like this.
Round 1
You spend your slow action to assume overwatch position and you select a hex. You can't use it for now since you don't have any slow action left.
Round 2
Since you don't loose your overwatch position, by doing nothing, you are still in overwatch position, but now you have have a slow action to save and this means that you can use that slow action for an overwatch fire with your telescopic sight.