While the front halves of these cards are fine, the reward for flashing them back is great enough that they see play despite their front halves (although this sometimes requires a specific deck or synergy, like Madness for Roar of the Wurm or Gifts Ungiven for Unburial Rites)
See section 5, Turn Structure. 2
There are 15 to collect: nine normal cards and six foil
Shadowless cards were printed during the initial run of Base Set before the drop shadow was added to the card border in subsequent print runs, making them identifiable by their slightly different border design and, most importantly, the absence of the shadow that gives the variant its name
You will get these packs if you participate in a gym battle at a local game store in Japan
BPhillipYork: This enables a whole deck around discarding and replaying, essentially giving madness to everything in your deck, though, obviously it's mayhem