Marth Tech: Heartshield
Description / How To Perform
Heartshielding essentially functions as 
Powershield storage. For non-projectile attacks it can be activated by performing a frame 1 Powershield on an attack
that would deal less than 3% damage. This is the same for projectile attacks with the stipulation that you also must have the hitbox collide with your "powershield sphere" but not your "reflect powershield sphere". The reflect sphere is smaller, and is what makes
reflecting projectiles with powershield more difficult to time correctly than powershielding a non-projectile. Regardless, for as long as you hold your shield with a heartshield activated you will automatically powershield the next attack that connects with it.
Difficulty: 
(Master)
Usefulness: 
(Rarely Useful)
Extra Info
For more details about the heartshield and it's mechanics, this reddit thread by schmooblidon is a good resource.
There aren't many attacks that can be Heartshielded on reaction. An example of one that can, would be Falco lasers. Assuming that they were staled enough to deal 3% damage or less.