Use this in casual speech or a message when you want suspense before the condition: When you, when you wheeeen finally open that door, tell me what you see. The condition works the same as plain when you; the repeat and stretched vowel only add drama, so plain when you open that door is the neutral choice. It's informal spoken-style wording, so keep it out of formal or serious text.
Using it in a message
Use the pattern when you want suspense before a casual condition. The condition itself is ordinary, only the delivery changes. Try: When you, when you wheeeen walk out of that exam hall, call me right away. The repeat and stretched vowel delay the payoff, so the news lands harder.
You're teasing someone before opening a gift or a door on a call. Something like: When you, when you wheeeen open it, look at the tag first. Here the drawn-out wheeeen builds anticipation, while plain when you open it would sound brisk and neutral.
You're announcing plans in a group chat where drama fits the mood: When you, when you wheeeen get to the airport, send me a photo. The repetition works like a drumroll; the condition, arriving, is unchanged.
A contrasting context: you're writing a work email that includes a condition. Then keep it plain: When you arrive, please check in at reception. The repeated, stretched form would read as playful or joking there, and a serious email needs the plain version.
When to skip the stretch
One qualifier decides the choice: if the stakes are real and serious, don't stretch. When you, when you wheeeen sign the lease, read clause nine turns a warning into a bit, so a landlord or lawyer might miss the point. Keep the drama for low-stakes suspense and say it plainly when the outcome matters.
As for what the stretch adds, it seems to signal tone or emphasis rather than change the condition itself, but that's an informal pattern without a settled rule, so test it on a listener before trusting it in anything important.