I tripped a 20amp breaker but didn't feel a shock - why?
Power doesn't usually go through you

Because you're a lousy conductor.  Also if you possess even the slightest amount of fieldcraft, you habitually and perhaps unconsciously position yourself so you are not a current path. 

Electricity is not the black oil from X-files.  It doesn't seek out human skin, it will seek out all possible current paths to return at the same time. If you're contacting L1, then neutral, ground or L2 are returns.  L1 is not a return.   

Current will take each path in proportion to its conductance (which is 1/resistance, Siemens = 1 / Ohm ).  So typically you have shoes on, and are not leaning on a grounded panel, and have not carefully attached a grounding strap to any fresh piercings, so your conductance is very low (megaohms/microsiemens). Meanwhile those other wires have extremely high conductance (milliohms/kilosiemens). 

So one of two things happens, either so little current flows through you (microamps) that you don't feel it, or a dangerous current (milliamps) flows through you.  And then hundreds of amps also flows through the wire (as evidenced by the arc flash) and the breaker magnetic-trips, interrupting the current flow in milliseconds and so you never get to find out if it's lethal.