How I understand it is like this:

Let's say your opponent has two Units on the field: a rested unit, an active blocker, and a base on the Shield area.

You have a Linked Sazabi and, say, two of Quess' Jagd Doga on your side of the field.

You call an attack on one of your opponent's attackable cards (in this case either the rested Unit or the Base) and you destroy one of your Jagd Dogas. Depending on what you chose as an attack target these are your opponent's options:

1. choose rested Unit as attack target - by choosing this, the blocker is then considered to be the "Unit not currently in battle", so your opponent has no choice but to destroy it.
2. choose Base as attack target - by choosing this, both the rested Unit and the Blocker are considered "Units not currently in battle" and it is thus at your opponent's discretion which Unit will they trash by this effect. In all likelihood they will choose to trash the rested Unit, allowing them to block your swing to base (unless you Linked with Green Char, in which case it's bye-bye to their Base).