It has the text: "If an effect would put one or more tokens onto the battlefield under your control, it puts twice that many of those tokens onto the battlefield instead. If an effect would place one or more counters on a permanent you control, it places twice that many of those counters on that permanent instead."
Seems simple enough. Here are some statements. Can you figure out for each one if the statement is true or false? All of the statements assume that you control a Doubling Season.
1) You control a face-down Hooded Hydra, and you turn it face up by paying its morph cost. The hydra will only get five +1/+1 counters.
Hooded Hydra says: "As Hooded Hydra is turned face up, put five +1/+1 counters on it."
2) You control a face-down Hooded Hydra, and your opponent casts Break Open targeting it. It will get ten +1/+1 counters.
Break Open says: "Turn target face-down creature an opponent controls face up."
3) You control a Words of Wilding, and you activate its ability during your upkeep, and proceed to the draw step. When you would draw your card, you instead put a 2/2 green Bear creature token onto the battlefield. Only one.
Words of Wilding says: "1: The next time you would draw a card this turn, put a 2/2 green Bear creature token onto the battlefield instead."
4) You control a Words of Wilding and an Alchemist's Apprentice. You activate the ability of the former, and then the ability of the latter. Instead of drawing a card, you instead put two 2/2 green Bear creature tokens onto the battlefield.
Alchemist's Apprentice says: "Sacrifice Alchemist's Apprentice: Draw a card."
5) You play a Vivid Grove as your land drop. It will enter the battlefield with just two charge counters.
Vivid Grove says: "Vivid Grove enters the battlefield tapped with two charge counters on it."
6) You control a Sakura-Tribe Scout, and you use its ability to put a Vivid Grove from your hand onto the battlefield. It will enter the battlefield with four charge counters.
Sakura-Tribe Scout says: "(Tap): You may put a land card from your hand onto the battlefield."
It may be a surprise to many players, but the answer to each one of the statements above is "true".
Why does Doubling Season sometimes double the number of counters/tokens, and sometimes it doesn't? The reason for this is quite obscure and technical.
The key word here is Doubling Season's wording "if an effect would put..." It's also related to a technical aspect of replacement effects.
The word "effect" in the card's text is not just random or inconsequential. The rules of the game explicitly define "effect" as meaning "spell or ability". In other words, what the text in Doubling Season is really saying is, effectively: "If one or more tokens would be put onto the battlefield under your control as a consequence of a spell or ability..." (and the same for counters).
Turning a face-down permanent face-up by paying the permanent's morph cost is not activating an ability. Instead, it's a special action (this is explicitly specified in the rules), and special actions do not count as activated abilities. Hooded Hydra's "As Hooded Hydra is turned face up..." is a replacement effect (as specified in the rules), and replacement effects do not change the nature or type of the event they are affecting; they simply modify the event in some way. Thus the reason why the hydra is getting counters is a special action, not a spell or ability, and thus Doubling Season has no effect on it.
If the hydra turns face-up due to a spell like Break Open, this changes things. Now it's a spell that's turning it face-up, and thus Doubling Season now has an effect on it.
Drawing a card at the beginning of the draw step is not a spell or an ability either, but a so-called turn-based action. A replacement effect like that of Words of Wilding doesn't make it anything else. Even though the event was modified, it's still a turn-based action, and thus Doubling Season has no effect on it. Thus you get only one bear token.
However, if you are drawing a card with an ability like the one on Alchemist's Apprentice, this once again changes things. Now it's not a turn-based action, but a resolving ability that's making you draw a card. Thus Doubling Season affects it (or rather, the modified version of it by Words of Wilding).
Likewise playing a land is also a special action. It's not casting a spell nor is it an ability. This is why playing a vivid land as your land drop won't double the counters on it. However, putting a land onto the battlefield due to a spell or ability (like the one on Sakura-Tribe Scout) does double the counters.
(Note that this is a consequence of the rules that may change in the future, as the rules coordinators have expressed the sentiment that they may want to change this so that it becomes more intuitive. The rules work like above as of writing this blog post, but may become different in the future.)
One of the reasons why Doubling Season is worded like that (ie. "if an effect would put..."), and why it's so difficult to change so that the abovementioned interactions would become more intuitive, is that they don't want it interacting with things like costs that put counters on permanents (such as activating planeswalker abilities).
