たとたん: an unexpected change the moment an action occurs
Use たとたん for an event occurring at the very moment another action takes place. Distinguish it from a simple sequence and from describing a planned action.
A change at that very moment
窓を開けたとたん、冷たい風が入ってきました。 means that a cold wind came in the moment the speaker opened the window. It presents the immediate connection between the action and event as a vivid scene.
The preceding verb takes the ta-form, and とたんに is also possible. 椅子に座ったとたんに、電話が鳴りました。 means that the phone rang just as the speaker sat down. It does not assert that sitting down caused the phone to ring.
Comparing a simple sequence with てから
窓を開けてから、机を拭きました。 gives the order: the speaker opened the window and then wiped the desk. てから alone does not say the interval was almost zero or that wiping the desk was unexpected.
The basic use of たとたん introduces an actual, sudden event or change rather than the speaker's planned action. For a plan, 家に着いたら、電話します。 naturally links arriving home with an intention to call.
Sudden timing is not the same as causation
薬を飲んだとたん、眠くなりました。 emphasizes becoming sleepy immediately after taking medicine. Context may suggest a cause, but the grammar itself does not prove a medical causal relationship.
A mental change can also follow, as in 彼の名前を聞いたとたん、昔のことを思い出しました。, meaning that hearing his name immediately brought back memories. The expression can introduce an unexpected reaction, not only physical movement.
Quick check: Does 立ち上がったとたん、めまいがしました。 mean the speaker stood up deliberately to become dizzy?
Show the answer and why
No. It says that the speaker felt dizzy at the moment they stood up. たとたん links an immediately following event; it does not express the action's purpose.