hi.
this is my understanding:
1, it comes after the verb,means do something once or give a try. (mostly used)
eg,
I gave a glance at her.我看了一下她
Let me take a look.让我看一下.
Can you explain it?你能解释一下吗?
Please introduce your self.请介绍一下你自己。
2, in a very short period of time
eg,
I am shocked.我一下惊呆了
I immediately recognized him. 我一下认出了他。