Power-Up Stacking is a very useful tool

When you collect a Power-Up, whether its Air, Earth, Ice or Fire, Tux can collect another of the same Power-Up, and granted he hasn't lost the previous one, can stack the two together.

Fire Flowers:

Fire flowers when stacked give an extra bullet to be on screen at a time.

Air Flowers:

Air flowers when stacked give 0.5s extra glide time (This is slightly random and a bit variable, we're not exactly sure how this works still...)

Ice Flowers:

Ice flowers when stacked give an extra bullet to be on screen at a time.

Earth Flowers:

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Power-Up Swapping:

When you have, lets say, a tier 5 Air Flower (an Air Flower stacked 5 times). Granted you haven't taken damage, you can collect a Fire flower. You can then chose to stack the Fire Flower a bunch, if you wanted. If you then would go back to collect another Air Flower, the previous stack you had, 5, is kept, and you also receive the bonus tier you just collected, Granted you haven't taken damage whilst swapping them. This is very useful in 100% speedruns on newer versions, and give the game a lot more depth when it comes to routing.

Fireball + Glide mechanics:

You can reverse a fireball and manipulate its trajectory based on Tux's movement. For example, if you're travelling at full speed, reverse and immediately throw a fireball, its trajectory will reflect the opposite effect of the velocity of Tux. You can see an example of this here:

Glide mechanics can be manipulated by a technique I invented called "hover stalling" or "glide stalling" This is where while airborne, you repeatedly tap jump to a specific rythm. Finding this rythm is kind of weird. This is especially useful in the newer versions, especially in "Ice in the Hole"

PM on discord if there's more i should add.