Motorshield: cutting the Vin jumper

Hi! I am using an Arduino Motorshield V2 to power 2 stepper motors. I want to cut the Vin jumper, but whenever I remove the connection, the power button (and every other blinking light on the Arduino) goes dim.

Note: I do have it hooked up to an external power source (and the light on the power source is on, so I'm assuming that's working properly.)

What am I doing wrong? Am I missing a step after cutting the Vin jumper?

Thanks for your help!

What are doing wrong? You are cutting the Vin jumprr, that's what. Why would you do that ?