Nothing but that wasn't the sentence we were asked to translate. Duo isn't asking for a 'similar' translation or 'something that makes sense'. You have to translate the German as closely in meaning as possible. Otherwise, this wouldn't be a very good language learning website. Your answer includes an imperative (let's), whereas the German is just a statement of fact; "we're jumping on my signal".
Thinking about it, a better (literal) translation of this sentence into English would be: "We're jumping on my signal" which again, is a statement of fact not an imperative.
From CatMcCat's comment, it appears Duo also accepts the imperative "let's".
If you're referring to "On my mark, jump" - that wasn't meant as a translation of Duo's sentence but rather as a correction of CatMcCat's translation (on my mark, let's jump) which was incorrect. There is a fixed English expression: "On my mark, jump" (which includes the speaker as well) otherwise, you would have to say; "Let's jump on my mark".
In order for this to be first person imperative, would the adverbial phrase have to come last? "Springen wir!" would be "Let's jump!"; however with an adverbial phrase at the beginning, the verb and subject are already transposed since the verb must always be in second position, so does that simply make it ambiguous whether it's indicative or imperative, or does it make it only indicative in this instance?
Basically, what I'm asking is this: does this sentence mean example 1, 2, or either?
- "Wir springen auf mein Zeichen." (We're jumping on my signal)
- "Springen wir auf mein Zeichen!" (Let's jump on my signal)
Why should it be? Since there is just one verb here (springen), and it's not imperative, it should be in the second logical position. The standard order would be "Wir springen auf mein Zeichen" - "we jump on my mark". However, if you want to stress "on my mark", you can move it in front (just like you would in English). Now, "on my mark" - "Auf mein Zeichen" - is a single logical unit here, and it must be followed by a verb in German since the verb has to occupy the second logical position. Hence "Auf mein Zeichen springen wir!"