Double-Steering is an exploit left over by EA Black Box in most of their entries in the Need For Speed franchises. It combines the same bind on steering for analog and digital controls at the same time. In most of their games pressing both of them at the same time either does nothing, slows your car too much or, in NFS Underground 1's case, it helps you steer/drift way faster than normal
It requires a relatively low input over your analog input (anywhere from 0 to 35; depends on car) and, while you hold that value, you press digital once, multiple times, or hold it. Each FWD car has an unique style of doing it, most basic type of DS being on the Acura Integra.
Cars like the Integra or 206 don't really need any artificial help but for the other ones, not having the right angle is life or death. You can use DXTweak or any app from your controller/wheel to limit your inputs or a DIY method using rubberband tangled around your joystick
Double-Steering in FWD Cars, compared to RWD, gives you unlimited control over every action you do to your car. You have to keep it drifting manually and more freedom in your actions.
Just turning your analog stick and pressing digital results in a normal DS. As you progress doing it, you will realize on your own that in some cases you can lower on your input or the amount of times you press the digital button for smoother and faster lines
SDS is a mechanic where you start with a normal DS and then you just hold digital. That results in a drift where the car gains more angle in a controlled manner. On asphalt, you will need some inertia to do it, meanwhile on sidewalks, cobblestone or dirt you can do it directly. It is very useful for wallbounces or wallrides, to make your car hit a wall with the back.
This is when you initiate a DS at a low analog angle and keep holding it. You know you did it right if you leave only a tire mark on the road or 1 tiremark is way thicker than the other one. It is good on loose surfaces to keep a good racing line and keep speed. Only in the Neon, this makes you accelerate a bit faster.
This is a very quick method to gain speed over any elevation change. You just flick the stick and press digital over a bomb and you gain more speed.