This question was posted on the cams page of the mechanism pages. It's interesting so I thought I'd re-post it to the blog where more people would see it.
A cam is, at its simplest, a surface describing a movement, all wrapped round in a circle. The highs and lows of the surface create the highs and lows of the cam follower's path.
To create a flat area in a cam, an area where the cam follower doesn't move up or down, you need an area that follows the circumference of the circle centred on the centre of the cam.
In this cam there are two quarter turn flat areas. In a mechanism this would give you a cam follower that stayed high for a quarter turn, drops down, stays still again for another quarter turn then rises again. It should do what you are after.