Ask one thing
六爻納甲
Six tosses of three coins build a hexagram. State your question first — the reading is bound to it.
Read the moment
奇門遁甲
The board is built from the current hour. It answers a direction question: where should this be approached from, and when.
One question, one hexagram
梅花易數
No coins and no board. You bring a question and a way to cast it — the moment, a couple of numbers, or what you just heard — and the hexagram is built from that.