Credit to Baxter for creating this chart.
Each enemy killed increments the kill counter. Starting at power on, the kill counter starts at 0. All enemies killed increases the counter by 1. If the counter is 0 and an A group is killed, a rupee has a chance to drop. After this kill, if the next enemy killed is in the D group, a fairy has a chance to drop.
Example: Game is powered on.
Red darknut killed, drops bomb or no item because of B group (counter is at 1)
Two keese are killed, no drops because of X group (counter is at 3)
Stalfos is killed, drops 5 rupee or no item because of C group (counter is at 4)
Forced item drops override regular drops.