Quick question, If a character mentions ahead of time that they have backup and later use that card during an incident with another character to save their skin by "ambushing" the character and disabiling them does that count as powerplaying.
note: The character was not controlled at any point but rather was blindsided by an NPC.
Technically, by your own words, you (as a player) controlling an NPC (or Non-Player Character) is illegal anyways. This would be an example of God-Modding one of the Author's (my title for the person running the RP) characters that they use to populate the world. But to be more concise to your question...
By saying you have backup, this would assume that you are really trying to play 2 characters. Now, this may be allowed by the Author, but it has to be accepted by them. If you just put "I have backup waiting" in your character app but never explain it, never ask the Author if this is okay, never add in the application for your second character, I can completely understand an Author's reluctance to accept your backup. This is because you are suddenly two characters, which is obscenely powerful and unnecessary for the RP.
Specifically for your question of ambush, this isn't power-playing so much (as power-playing is primarily the control of another player's character through your own writing which
MAY show up depending on how you write the ambush, but the situation itself is not power-playing) but more so God-Modding. You are effectively saying, without the Author's control and direction, that the world is
in fact like this and you write your own story. Characters are not where the Author imagines they will be, situations develop outside of the Author's control, and you are now the one controlling the RP. This would be God-Modding by definition, even in a small situation.