Standard circuit breakers are not designed to trip instantaneously for all loads.  Breakers are designed with time curves they must meet, and for anything but the highest currents there is a noticeable delay from seconds to hours.

[Schneider Electric PDF](http://static.schneider-electric.us/docs/Circuit%20Protection/Molded%20Case%20Circuit%20Breakers/0100-400%20A%20Frame%20FA-LA/FA-FC-FH/0600DB0105.pdf)

[![enter image description here][1]][1]

I've marked an example on the chart which shows that at a current of 15 times the rated current, it can take up to 2 seconds to trip (download the PDF and zoom in so you can actually read all the numbers).  For a 15 amp breaker, that's over 200amps for almost 2 seconds.  That's enough power to melt strands of your extension cord and possibly damage whatever you used to short the first two outlets.

When you touch two wires, only a small area of the wire touches, and explosively heats and melts (the POP and spark).  I suspect if you stripped the extension cord and used a proper wire clamp on the ends to make a good connection, you might be able to pull enough current to almost immediately trip the breaker, assuming the outlet doesn't internally melt when you plug it in.

**The main point is that this IS NOT a good thing to do.**  Tripping a breaker to find it with a short is pretty dangerous, and you can damage outlets or other wire junctions in the process.  If there's a wire nut in an adjacent outlet that's not making the best connection, it could melt.  Then you have to find and fix that which could be a challenge unless smoke and fire direct you to the problem area.

Plug in a radio, turn it up, and start manually throwing breakers.  When the music stops, you found the right breaker.  Don't purposely short wires.

  [1]: https://i.sstatic.net/xO9mI.png