The main purpose of the down-and-up stunt is to confuse the QB into thinking one thing, when you are actually doing another. This is nothing new to defensive football, however I rarely hear of teams utilizing this technique against the zone read.
In the beginning, our DE's would honor the block-down-step-down rules to a tee, when attacking zone read teams. However, we found that sometimes the QB was a better runner than the RB. The first thing we did was add a switch in responsibilities with the DE and the LB. This was identical to the squat stunt, the DE came off the ball, and just squatted waiting for the QB to pull the football. The LB behind him, could them come down inside and chase the RB. This wasn't too bad, and worked at first because teams who played us were used to seeing our DE chase. What happened was we faced some good tackles that could cut off our LB's, and we also found teams started trapping our "soft" end and still having the QB keep it. So this was the beginning of the down and up. We had to find a way to "trick" the QB into pulling the football, yet keep our DE there to play the QB. We also needed to be able to get hands on the OT, to keep him from getting a free release to the LB. In short, this is how the down and up was born. Now lets look more into the technique of the stunt.
The technique involved in the down-and-up stunt, was exactly the same as that of the DE's reaction to a downblock. We taught our DE's, on a downblock read, to read down the line to up the field. Basically looking for a puller, or kickout block or ballcarrier from the inside. When we ran our squat stunt, the DE would never see the pulling guard until it was too late. Now, by having him still get hands on, he could see if there was a puller coming his way. If so he was still able to engage it, and wrong shoulder the puller (a term we called splatter). What the DE did was, get hands on, but not as aggressive as he would vs. a normal down block. He would still read down the line to up the field, and upon seeing no puller, he would set his feet and drive vertically into the backfield. This technique got the DE's shoulders turned, which to the QB looked like a pull read. However, the DE at the last possible moment, would disengage the OT and come upfield for the QB.
To clarify, this was just a stunt we used, not something we sat in all the time. However it gave us three ways to defend, at the time, one tough running play. It made life miserable for zone read QB's, and forced a lot of teams out of their base play. I know zone read football isn't what it was just a few years ago, however, this is a very good stunt to have in your arsenal if you run up against a team that is very good at running the zone read play.