I have to question 12...c4 - isn't that sort of move, removing the central tension and allowing white a free hand on the k-side, usually bad?

Spielmann really seemed to have gotten a winning k-side attack, but threw it away with inaccurate play.

That is well said.

<I have to question 12...c4 - isn't that sort of move, removing the central tension and allowing white a free hand on the k-side, usually bad?>

So the books say...I think it tends to work better than its reputation.

Vukovic wote:

<Indirect defence [that is, defense by counterattack in a different part of the board] by means of advancing a pawn majority on the queenside was at one time held in high esteem and was more often practised than nowadays; the real reason for this is that we have acquired a better understanding of the centre and the technique of centralization, thanks to the hypermodern school. It is clear that the player who has a majority on the wing, when the material is equally balanced, will usually not have the greater influence or pressure in the centre, nor will he have the necessary conditions for a central action. A majority on the wing is created at the expense of the centre and means giving up lateral pressure on the centre, and that is the sort of strategy which the masters of today are reluctant to adopt. The struggle for the centre is a characteristic feature of all present-day openings, and the centralization of the pieces is an important theme of modern positional play; as a result, a majority on the wing appears nowadays more as a by-product of a particular strategy than its primary aim (for example, in the Exchange Variation of the Gruenfeld Defence).>

Tarrasch vs Schlechter, 1896