How to reduce altitude in a passenger jet without gaining forward speed in an emergency?
Ah that's a good question indeed. You asked 


  How to reduce altitude in a passenger jet


Normally most of the passenger jets have spoilers They are effective and can be deployed at any speed.

Now what happens in case you don't have spoilers for any reason. I was discussing the subject with a pilot many years ago and he told me the trick you mentioned: you apply rudder to one direction (I assume opposite to the engine rotation to avoid unpleasant surprises). That increases dramatically the forward surface of the plane against the relative air flow. That results in increased drag and thus speed loss.

But yaw causes roll (and vice versa). Also the wing of the side you are yawing to, has less lift because it is shaded from the forebody of the plane. This is where the aileron is needed. You need aileron to maintain lateral stability and prevent the aircraft from rolling. See the image bellow. The orange lines denote the area of increased drag by the fuselage. The green area is the "shaded" area of decreased lift of the wing.



Top view from wikipedia. Derivative work by me.

According to that pilot, this is more effective than spoilers. But there is a trap: You need to be careful not to yaw the plane too much because the shaded wing will loose too much of its lift and will result in capsizing the plane.

EDIT: Finally, you mentioned emergency. There are different types of emergencies. Pressurization is an emergency situation but does not affect aircraft control (at least not directly). Hydraulics problems or engine loss is an emergency as well. But that will affect the way you will handle the aircraft. Here I cannot help you. A trained pilot perhaps can answer, though I believe that the answers will vary depending on each planes flight manual.