The sentence is:
MC is revealing a secret how to win every dice game. But I'm not sure the meaning of his words. Does it mean throwing dice one by one?
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ひとつ（で） actually applies to the previous word 投げ方 here. It's kind of like だけ in this usage; he is saying to become able to control the result of the dice solely through just the 投げ方 (way of throwing).
In the above sentence, the 投げ方 is described as the sole deciding factor for producing the result.
Wiktionary has a good example of the usage:
ある物事を左右する要素。 (The factor that influences another event.)
君の返事一つでどうにでもなることだ。 (All it would take is) the right reply from you and this could all go away.
That's not to say that there aren't other ways the situation could be resolved, but a reply would serve on its own to decide things all by itself.
大辞林 also highlights its use in this kind of situation.
（ 名 ）
⑥ そうすることによって決まる，それ次第であることを強調していうときに用いる。 「やるかやめるか決心ひとつだ」 「心の持ちようひとつでどうにでも変わる」 (used to emphasize that sth will be resolved/settled by doing X; or hinges on / is dependent on X). "To do it or not -- it is all a matter of how you will make up your mind." "You can be whatever person you want to be; it all depends on your attitude."
大辞泉 is more plain about it and says not much more than だけ, but I get the impression that it's stronger than だけ and gives a greater sense of exclusion.
９ 名詞の下に付いて、それ以外にはないことを強調する語。…だけ。「身ひとつで脱出する」「母の手ひとつで育てられる」「行くか行かないかは自分の決心ひとつだ」 (Following a noun, emphasizes that there is nothing else than that. ...dake.) "Make it out using nothing more than one's own body" "Be raised solely by the hand of one's mother" "Whether to go or not entirely hinges on your own ability to decide"
(translations are mine)