Help with the use of 一 in those sentences

[Alhazred]

Can anyone help me with the use of 一 in those two sentences please? I think it means that once something is done or a certain condition is achieved, fulfilled whatever, then there is a consequence. Is that right? For example, I'd translate the first sentence as: He became happy and stood up to sing. And the second one as: Once he was done speaking, he left.

他一高兴，就唱起来了。

他一说完，就走了。

A short explanation on that use of 一 would be most welcome too.

Thanks in advance.

[nipponman]

I would say your assement is correct.

[roddy]

I think it's more a way of emphasising that as soon as A happens, B happens. So for example,

他一说完，就走了 isn't just 'He finished speaking and then left', but more 'As soon as he finished speaking, he left' - ie, it happen quickly, with nothing in between.

Roddy