This is the combination of a little documented feature of V8 mixed with a non-documented feature of the node.js debugger client. Say you have an already running node process that you want to debug. The most common scenario is a process that’s running wild on one of your testing environments and you want to see what’s wrong with it. So a backtrace would be helpful. And full debugging even more. The following should help:
# start the remote debugger in the already running node process
kill -s USR1 pid
# attach to the debugger using the node.js client
node debug host:5858
# see where you are
From there you can poke around. You can also continue and pause again to see if you seem to consistently end up in the same code area. Or write a script that regularly connects, dumps the backtrace and disconnects.