Carbon brushes deliver power to the rotating part of the motor - they rest against a segmented cylinder on the rotor of the motor called a commutator. As each segment of the commutator passes under the brush it's quite normal for a small spark to jump across the gap.
You can see a home made electric motor sparking away in this video...
howtomendit.com for free repair help, information and advice.
Try fixfault.com to fix jigsaws.