Marth Tech: Amsah Tech
Description / How To Perform
When grounded and hit by an attack that sends you at a low enough angle, 
DI and/or SDI can be used to cause you to connect with the ground before being launched. Since you collide with the ground, the attack can then be Teched. For some attacks, DI'ing down and forward/away is enough to collide with the ground, but there are many others where holding the c-stick down to ASDI down as well is necessary.
Difficulty: 
(Advanced)
Usefulness: (Extremely Useful)
Extra Info
It's usually better to DI down and towards the center of the stage rather than away so that you'll stop sliding at the ledge. Holding away will cause you to slide off the stage instead.
One of the most common and useful applications of Amsah Teching is for surviving recoveries where you land on the stage with Upb. You can preemptively prepare to Amsah Tech the attack they'll use to punish your landing lag. Lower level players won't be ready for you to tech like this, and also won't react in time to respond with an additional attack.