。、・・・。
I tilt my head. Not this pawn, but this knight...
Literal
I [topic-は] neck [object-を] twist. This pawn [copula-ではなくて], this [of-の] knight [object-を]...
Chess-game internal monologue. 首をひねる 'to twist/tilt the neck' is a set phrase for being puzzled or deliberating a decision. ~ではなくて is the negative copula-te form 'not X, (but) Y,' with the alternative implied by a trailing thought. The ellipsis after ナイトを... leaves the verb (probably 'move') unsaid, representing the speaker's ongoing deliberation. The chess terms ポーン and ナイト are loanwords.