Guardian Acorn, Piece of Power and Tracy's Medicine each use a kill counter, which is a number that increases for every enemy you kill.
A Guardian Acorn is a powerup that drops from enemies very frequently in Link’s Awakening. It makes you take half damage for a certain amount of time. Due to the fact that we already do not want to be taking damage and that it takes time to pick it up, it is regarded as a pretty universal time loss. It could be useful in Hero Mode, since you cannot heal, it would allow you to perform tricks that require you to take damage for a lesser cost, reducing required deaths, ultimately gaining time. Fortunately for most, it is relatively easy to manipulate the Guardian Acorn as its kill counter resets every time you take damage.
The Guardian Acorn will drop every 14 kills without taking damage (Possible exceptions: Dying to the Shopkeeper doesn't reset the Acorn count, being hit by a Bone Putter will reset the Acorn count despite you not taking any damage). More details on that in the Piece of Power section.
A Piece of Power is a powerup that drops from enemies somewhat frequently in Link's Awakening. It gives Link +1 Sword damage, 25% increased movement speed, and extra knockback on his sword hits. The runner is able to manipulate when it drops with some careful and on-the-fly routing.
The formula for how many kills it takes for a Piece of Power drop is ( 32 + Total Heart Containers ). At the start of the game when you have 3 total heart containers, this means that a Piece of Power drops every 35 kills. Later in the game when you might have 5 total heart containers, they will drop every 37 kills. You can use this formula to your advantage to get a Piece of Power drop when you want it, such as before long boss fights like Slime Eel.
There are some intricacies to this manipulation that you should know:
Due to the animation and text box that comes with picking up a Piece of Power, it is best to avoid picking one up in places that you don't explicitly need its effects.
Tracy's Medicine is an extremely useful item that powers Tracy Boosts. In her house, Tracy will sell her medicine to Link at one of two different prices: 28 rupees or 42 rupees. This price is set by whether your current kill count is an even or odd number. As such, it is possible to manipulate what the price is by knowing your total kill count.
An exception to the above is that on every 8th purchase of Tracy's Medicine, it will only cost 7 rupees.