How do you make a dovetail cut to patch a crack in concrete block?

If I have a horizontal crack on a concrete block foundation and I am told to make a dovetail cut prior to applying the cement how would the dovetail cut look if I am facing the block from the front ? Do I chisel above and below the crack ?

When you say backcut, does this refer to reversing the angle of my cut when using a grinder ? Also my cracks are at the joints, is a dovetail cut still possible ?

You would chisel out the crack the width of your chisel (about an inch or so) and about 1/2 inch deep. You then backcut the chiseled area to make a groove that will hold the cement. This picture illustrates what I'm talking about.
http://www.kighthomecenter.com/build_tip...
When I did this, I used a chipping gun to make the initial groove and an angle grinder with an abrasive disk to make the backcut.

as long as the crack isn't compromising structural integrity, the joint is the best place for this. just take an angle grinder and run along the top and bottom of the joint. all you need to do is make the groove wider at the inside than the front but don't go too deep, if you knock out all the joint, you wont have any backing to point your mud against and you'll just be pushing it down inside the wall. so, grind out a little into the bottom of the top block, and the top of the bottom block but not too sharp cause then the edges of the block can break off. if its not too long a crack, you can just use your fingers and pack it full of mud (mortar) you're gonna want the mud to be pretty stiff. if you have a lot to do, get yourself a tuck pointer, its not very expensive. then strike it off and you're done.