Everyone starts somewhere, I get stuff horribly wrong all the time.
Throws don't have hitlag and hitlag makes it possible for ASDI and SDI. Jigg's f-throw has hitlag and therefore, makes ASDI and SDI possible.
What you are seeing is Regular Directional Influence. Regular DI influences the original trajectory, so yes, it does bend the actual angle.
What Magus is talking about is still Regular DI, if Peach f-throws you and it take X amount of frames to complete, you need to (should) hold upwards on X-1 frames in order to influence the trajectory upwards.
What you were thinking of is Smash DI and Automatic Smash DI, which can also dramatically help you survive stuff. However, throws aren't affected (I wish they were though.)
ASDI allows you to change your position during the final frame of hitlag (which is nonexistent during throws,) and it gets it's input from your D-stick (hence automatic.) If you get F-smashed by Marth and you hold up, you get Regular DI which influences your trajectory to be higher than normal and you get one input of ASDI, and since you were holding up, you're character shifts himself slightly upwards and takes the hit slightly higher, making the F-smash not as brutal.
The C-stick can be used to out prioritize the D-stick in certain situations where you want to perform Armada techs or double-stick teching, but you'll learn that at a later time. I don't want to confuse you yet.
SDI is the exact same thing as ASDI, but goes farther and instead of getting only one input at the last frame of hitlag, you can input a direction for EVERY frame of hitlag (pretty broken eh?) however since we're just mortals we'd be more than fine with just 1 or 2 inputs.
All you need to do is hit these points with your D-stick during hitlag and you will go in that direction during that frame.
(credit to Magus)
What this is useful for is during edgeguards, if you aim for the ledge and get hit by something long like Marth's sword, you can use SDI to inch yourself towards the ledge and enable a ledgetech.
You can also use it to drastically affect the trajectory of things. You don't influence the trajectory per se, but you change where you get hit, which really matters. For example, if you kneed while grounded, you go at a much harsher angle, however if you SDI upwards and take it airborne, you would dramatically improve your chances of survival.
SDI can be used in tandem with ASDI to avoid stuff like jab resets, but you can touch up on that subject later.