SDI is an acronym for Smash Directional Influence. The SDI can only be used during hitlag, which means that the time frame to do it is very small. SDI can be achieved by pressing any direction during hitlag, making you travel a short distance in that direction(and I mean teleport, since you move instantly in 1 frame every time you SDI). You cannot hold a direction before the hit to get SDI on some of the frames. The main point of SDI is to compensate as much as possible for the knock back you are gonna receive after the hit simply by hitting in an opposite direction. This would mean that if you get hit by an attack like fox's forward smash, which has a near horizontal knock back. This would mean that if fox sent you to the left, you would need to go to the right this way when you get the knock back you are actually farther from the DZ(death zones around the screen). It is important to note that you cannot SDI onto the ground to force a tech when you're on the ground, but in the air, this is very doable to reduce knock back. Since there are multiple hitlag on all of the attacks in the game, it is technically possible to SDI more than once in a hit. However since holding a direction does not count as an actual input for SDI, if you want to SDI to the left twice, you would have to press left, let go of the joystick, then press left again within a very small time frame. This can therefore be very tricky, but since SDI only registers new input there is a way to avoid such impracticalities by doing QCDI.