A great many of us are not happy with the issue divisions among the two major parties. Sadly, many feel they are throwing away their vote if they don't use on a (D) or an (R) even if just to make sure their vote helps block the opposing major party.
A second choice candidate is simple enough to implement. For every choice on the ballot you select a primary and secondary choice. If at any point in time your primary selection has no numerical chance to win your vote is applied to your secondary choice. So someone who supports the green party might choose the green candidate as their primary and the democrat as their secondary. If it comes down to (D) and (R) their votes still impact the election but at least their real views can be expressed.