A verb moves downward, and you want to mark whether it ends up near you or carries on down past you.
序 · The move
1Set the verb, then 下 to lock the motion as downward.Is the action actually going down, not up or across?
2Add 來 if the motion lands near you, 去 if it travels on down and away.At the end of the move, is the thing here with me (來) or gone off below (去)?
例 · Examples
1大家到了可以找地方坐下來sit down — the body settles down here, toward the speaker's spot休息rest。你可以從十樓走下去walk on down — the motion heads down and away, off below嗎？
When everyone arrives, you can find a place to sit down and rest. Can you walk down from the tenth floor?
V下來/下去 is the 下 branch of that family, fixed to the downward axis; the general directional complement opens every direction (起來 up, 出去 out, 過來 across). 下 is the down-only case.
V下來/下去2 (aspectual: 停下來 / 說下去)
Here the motion is real and downward (坐下來, 走下去). In the aspectual sense nothing physically descends: 下來 settles an action to a halt, 下去 keeps it running on. Literal path vs figure of continuation.
你可以從十樓走下來嗎？(speaker upstairs) → 走下去 (the trip heads down and away from you, so 去, not 來)
請坐下去休息。 → 請坐下來休息。 (settling onto a seat where you are ends near the speaker, so 來)
他跳下。 → 他跳下來。/他跳下去。 (下 alone leaves the endpoint unmarked; add 來 or 去 to place it)
English packs direction into one word — "sit down," "walk down" — with no marker for whose vantage the motion ends at. Chinese forces the choice: 來 if it lands by the speaker, 去 if it leaves them.