Here's my personal view about this:
Down Strike is exactly the same thing as the Charge talent but vertically: The added damage is due to added momentum, nothing else.
The (arbitrary) 1 yard rule simply mean you need to have been in free-fall for at least 1 yard to get the necessary momentum for the added damage (just like Charge requires a minimum horizontal distance). Just like Charge, the size of the Adept OR the target is irrelevant; the only thing that count is YOUR momentum.
As for what happens if the 1 yard 'fall' end up lower than the target's head ---- Well, it is called DOWN Strike so the attack (whatever weapon is used) must used the added momentum and thus must come from upward; stabbing horizontally would not get any bonus from downward momentum.
THUS, the attack would need to be either:
- Striking the head or the shoulders of the target (if the target is standing upright) and therefore mean that the 1 yard fall had to begin at least 1 yard higher than the target's head.
- Striking (downward) the chest/belly of the target IF said chest/belly is sufficiently larger than the head/shoulders (i.e. otherwise the head/shoulders are 'shielding' the chest from a downward strike).
- Striking a limb extended far enough from the rest of the body as to, again, not be 'shielded' from a downward strike by the rest of the body
- Or finally, if the target is not standing upright (i.e. knocked down), any part of the body that is at least 1 yard away from the where the attack began.
So: windling or troll is irrelevant; they both need 1 yard to gain the necessary momentum (remember; falling speed is NOT affected by your weight) so, in my opinion, there should not be any differences on how the rule is applied between races.