Stroking a motor increase the displacement of the motor with out boring the cylinder walls.
IE you decrease the length of the connecting rod or you decrease the stroke (stroke is the distance between the center of the crank and the center of the part of the crank that the connecting rod bolts to.
this is a link to a site with a good picture... A is the Stroke.
http://www.motortecmag.com/archives/.../imageset.html
The reason SOME people say stoking a motor is bad is because it can screw up a "good" Rod/stroke ratio.
that's when you divide the length of your connecting Rod by the length of the stroke. If this ratio is too low or too high it puts much more stress on the connecting rods, pistons, and cylinder walls.
It's also been said that there are massive increases in torque depending on the R/S ratio however more and more people are refuting that claim nowadays.
like here
http://www.motortecmag.com/archives/...AUG010201.html
search for it on google and you'll find lots of helpful stuff.