Brushing (swiping) the tip across the face of the cue ball (rock) usually toward the outside of the CB like applying BH English at and during impact while the tip is being compressed by the force of the stroke to increase RPMs.
It is held by some that this increases the throw angle on the OB over a conventional straight stroke.
You said:
"...my stroke that kept touching that boundary ball..."
So you were swiping the rock (Black Jargon) and why you got more than 5 degrees of throw on the OB.
Some (PJ) hold that this is not possible for the contact time is so short that the CB escapes immediately which may be true for a straight through stroke without BHE swipe.
I find that what you are doing to get more throw is useful to know and use to send the rock around the table. LOL
Here is a thread that touches on the subject you are wanting to know more about.
http://forums.azbilliards.com/showthread.php?t=417113&highlight=swipe+stroke