When to use it
Use it when you agree with a colleague's idea and want to move it forward. “Building on” gives them credit by name, and then you add one concrete next step. It shows you were listening and that you're thinking, not just nodding along.
Example
Sam: “We should simplify the signup page.”
You: “Building on Sam's point, we could cut the form to two fields and see if more people finish it.”
In an email or Slack message
In a Slack thread: “+1 to Sam's idea. Building on it: we could cut the form to two fields as a first test.” The “+1” is quick; the second sentence is what makes your reply useful.