Post by Nulgar
The @ symbol in a conditional is a shorthand for "target=", so "target=focus", "target=mouseover", meaning the action should be performed on the focus/mouseover, if it exists.
You can also append a Wind Shear without modifiers, so that the button also works (and displays the icon and thus cooldown) on your current target when you have no focus or are not hovering any enemy.
/cast Wind Shear; Wind Shear; Wind Shear
The #showtooltip enables the icon in the first place (you can also use #show, but that will show only the icon, but no tooltip on mouseover)
I added the part so that the macro also skips a friendly focus or mouseover.
Note that the macro will skip the first part if you press Alt, but have no focus, so it'll attempt to cast the spell on your hover target, and only if you're not hovering any unit, it will cast it on your target.
For more on macros in general, see the Wowpedia articles Making a macro
and Macro conditionals
One last thing, but that's just playing around/saving macro space, the following macro behaves the same as the one above: