As Ian says, these contructions are pretty common. I tried to find a grammatical definition. The closest I could find was something like: Use of present pariciple to show parallel action. I wasted ten minutes trying to find an explanation. I met a friend walking down the street. I got sick drinking vodka. He lost his fortune betting on football. You could insert an understood WHILE / IN THE PROCESS OF right before the past participle if it helps understand.

This is a good question. This grammar structure is common, but I understand why it could be confusing. Both of these sentences tell you 2 things. a) He injured his knee. He was playing hockey. b) I was up all night. I was updating my blog. The 2nd half of each sentence gives you information about the first half, like an explanation. a) He injured his knee. (How did he injure his knee?) Playing hockey. b) I was up all night. (Why were you up all night?) Updating my blog. We combine these two things as a way to be more concise and to tell a story that makes sense. A real English professor can probably give you a better explanation, but maybe this answer will help you. Good luck!