节骨眼儿

tā zài zhè ge jiē gu yǎn r shang lái le

他在这个节骨眼儿上来了。

He arrived at the critical moment.

jiù zài zhè ge jiē gu yǎn r shang yuán jūn gǎn dào le

就在这个节骨眼儿上，援军赶到了。

At this critical moment, reinforcements came up.