They are called dowels. Really no "trick" to speak of...they are slip fit...so you can rock the engine a little bit to get the engine to slide off the bolts. The one thing that always helps me is to support the tranny (with a jack, blocks, or even a strap underneath it), when you raise the engine up (to try to slide it forward off the tranny). If you make sure the tranny is still "in line" with the engine, it won't be binding on the bolts and dowels, when you try to break the engine loose from it. Then just work the engine side to side and it will slide off the bolts/dowels. Some engines seem to be "stuck" to the tranny a little harder than others, but they all will eventually come off.
Oh, another thing is to make sure the plate (the metal spacer between the engine and tranny) comes off evenly, when you get the engine broke loose. Sometimes it will hang up on the dowels and bolts and bind up the engine removal. Just make sure it either stays on the tranny or the engine and you will be fine.