T-Mar to the rescue!

It looks complicated in the diagram, but it is not so bad. I like these shifter when paired with an Altus rear derailleur because the shift lever has an overshift built in that puts the derailleur to the right place once you release the lever. And the Altus derailleur, while kind of lower end, shifts pretty well.

zandoval are you using this shifter with bull horn handlebars?