Salmond is able to game the regional system because the method of proportional representation that is used to determine the MSPs from the votes cast for each party penalises a party which has done well in the constituencies.
Therefore, if enough SNP voters follow the Salmond playbook and vote for Alba in the regional lists, then a SNP/Alba coalition will end up with more MSPs than the SNP would achieve if its supporters voted SNP in both the constituencies and the regional lists. In effect, assuming the SNP does well in the constituencies, a vote for Alba in the regional lists is worth more than a vote for the SNP, which is exactly what Salmond is counting on.