gistfile1.js

function handle_forms(event) {
    var $triggerObj = $(event.target)
    var num = parseInt($("#id_form-TOTAL_FORMS").val())

    var $last_row = $("#unidades_inline > tbody > tr:last")
    var $triggerObjRow = $triggerObj.parents("tr")

    if ($last_row.attr('id') == $triggerObjRow.attr('id')) {
        // than we are playing on last line, add new line if have something typed
        console.log(event,event.keyCode,'LAST_ROW',$triggerObj.attr('id'),$triggerObj.val(),event.target.value)
        if ($triggerObj.val().length > 0) {
        $("#unidades_inline > tbody").append(_EMPTY_INLINE_FORM_.replace(/__prefix__/g,num))
            $("#id_form-TOTAL_FORMS").val(num+1)
            $("#unidades_inline > tbody > tr:last input.trigger").bind('keyup',handle_forms)
            $("#unidades_inline > tbody > tr:last").fadeIn(600);
        }
    } else {
        // than we are playing on some other line
        console.log(event,event.keyCode,'SOME_ROW',$triggerObj.attr('id'),$triggerObj.val(),event.target.value)

        if ($("#unidades_inline > tbody > tr:last input.trigger").val() == "" && $("input.trigger",$("#unidades_inline > tbody > tr:last").prev()).val() == "") {
            // if two last inputs are blank, remove the last one
            $("#unidades_inline > tbody > tr:last").fadeOut(600,function(){
                $("#id_form-TOTAL_FORMS").val(num-1)
                $("#unidades_inline > tbody > tr:last input.trigger").unbind('keyup',handle_forms)
                $(this).remove()
                $("#unidades_inline > tbody > tr:last input.trigger").bind('keyup',handle_forms)
            })
        }
    }

}
console.log($("#unidades_inline > tbody > tr:last input.trigger"));
$("#unidades_inline > tbody > tr:last input.trigger").bind('keyup',handle_forms)