The reaction proceeds in two steps involving a redox change followed by complex formation:

1. Redox Reaction: When $KCN$ is added to Copper sulphate ($CuSO_4$), the $Cu^{2+}$ ions are first reduced to $Cu^+$ ions by the cyanide ions (similar to the reaction of $Cu^{2+}$ with $I^-$ described in the source). The unstable $Cu(CN)_2$ decomposes to give a white precipitate of Copper(I) cyanide ($CuCN$) and cyanogen gas $(CN)_2$. $2Cu^{2+} + 4CN^- \rightarrow 2CuCN(s) + (CN)_2(g)$
2. Complex Formation: In the presence of excess KCN, the insoluble $CuCN$ precipitate dissolves to form a stable soluble complex, potassium tetracyanidocuprate(I). $CuCN(s) + 3CN^-(aq) \rightarrow [Cu(CN)_4]^{3-}(aq)$

This specific reaction is highlighted in NCERT Exercise 5.14, asking for the coordination entity formed under these conditions . The resulting complex ion is $[Cu(CN)_4]^{3-}$, where Copper is in the +1 oxidation state.