I'm confused. How could you play Hack without spending ?

Chris: Once it's enchanting a permanent, there's no indication of which side was cast. It might even be that neither side was cast, due to Academy Rector or Nomad Mythmaker. But this wording allows the card to "remember" which side was cast.

Oh, because it's only a specific side on the stack. Right. Of course, you probably can't remember either...

One of the downsides of this direction for split Auras is that it requires no generic mana in the mana cost (resulting in decidedly un-splashable costs) but that goes against a major draw of split cards, that you'd situationally want to cast either half. To date, none of the 24 split cards (not even the Un one) have more than a single mana symbol of a given color on any side.

Can you do "When you cast Hack, put a hack counter on it. As long as Hack has a hack counter on it and enchanted creature is an artifact, you control enchanted creature"?

No, because it gets the counter while it's on the stack, then it promptly disappears when it resolves. If you're playing this in a situation where you have both red and blue sources on the board in such quantities that it's not obvious which side you played by looking at your mana base, feel free to put a counter on one side of the art as a way of distinguishing them.

Ah does it disappear? Shame. Well, you could be even nastier with the text and have "Put a hack counter on the permanent Hack becomes. As long as..." (as with the current oracle wording of Armor of Thorns).

I'm just trying to come up with a way to reduce the colour intensity for you.

This problem could be solved by turning the card into a flip card instead. While there isn't a precedent for unflippable flip cards, it would solve the problem of "what spell is this?" by utilizing the orientation of the card.

Also, I don't see why Hack can't just be an Enchant Artifact Creature... you know... besides the fact that its not as compatible with other cards.

The thing about flip cards is they have to have an unflipped side, which is the sole source of its characteristics when it's not flipped. That doesn't play well with cards like Persecute, Demand (of Supply // Demand), or Sanity Grinding.

The reason Hack can't have a different enchant ability is that a split card invariably has both sets of characteristics outside the stack, so that Slash would also have to comply with that enchant restriction. (Interestingly enough, you could still cast Slash targeting a nonartifact creature with that change, but then it couldn't enter the battlefield when it resolves, and goes to the graveyard instead by rule 303.4g. The same thing happens under the card's actual wording if you try to Hack a Sphinx of the Steel Wind, among other obscure possibilities.)

Point taken. If I had a choice between making a card that works in theory, or leaping through hoops to make my card match official rules, I'd choose the theory. Still, I can respect trying to work with the system instead of ignoring it.

That being said, if I worked for Wizards and I was a designer, and you were a rules manager, my next step would be to ask the art department if there was a way to make split cards be flipped in a way that makes it obvious that they aren't flip cards.