Garrett Hack replies: The most likely cause is your blade is flexing. The bed of the frog might be uneven, or the blade is sitting on some dirt or shavings so that it has less than full support. The lever cap might be too loose, allowing the blade to move around. Or there might be some twist or warp in your blade, so that it rocks on the bed. It might be simply that your edge is dull and digging in.
Joe Barry replies: The most likely cause is backlash. Backlash is a result of the clearances or slop in screw threads necessary for them to work. If there wasn’t some miniscule space between the threads, you would not be able to adjust the depth of cut without a wrench. What’s happening is that when the iron bites into the wood, it is being pushed back and up the inclined plane of the frog and taking out the backlash. The solution is to make your final adjust by bringing the blade down rather than up. That final turn of the adjustment screw downwards will take up the backlash.
Steve Branam replies: This happens when you plane against the grain. Think of planing like fanning out a deck of cards and running your hand over them. If you start on the top card and run your hand down the cards, it will pass smoothly over them. But if you start at the bottom card and run your hand up the deck, it will catch on their edges, lifting them and knocking them out of place.
The layers of wood that form the grain are the same, with the edge of the plane blade like your fingers. When you plane up against the grain, the blade catches on the edge of the wood fibers and is pulled down between them. This may also tear out a chunk. Reverse the direction of planing to follow the grain.
Sometimes the grain will both rise and fall along the same direction. These spots are more challenging to plane. You have to identify where the grain reverses, then plane toward that point from opposite directions.