code: function moveHandles ( upward, proposal, locations, handleNumbers ) {
  
              var proposals = locations.slice();
  
              var b = [!upward, upward];
              var f = [upward, !upward];
  
              // Copy handleNumbers so we don't change the dataset
              handleNumbers = handleNumbers.slice();
  
              // Check to see which handle is 'leading'.
              // If that one can't move the second can't either.
              if ( upward ) {
                  handleNumbers.reverse();
              }
  
              // Step 1: get the maximum percentage that any of the handles can move
              if ( handleNumbers.length > 1 ) {
  
                  handleNumbers.forEach(function(handleNumber, o) {
  
                      var to = checkHandlePosition(proposals, handleNumber, proposals[handleNumber] + proposal, b[o], f[o]);
  
                      // Stop if one of the handles can't move.
                      if ( to === false ) {
                          proposal = 0;
                      } else {
                          proposal = to - proposals[handleNumber];
                          proposals[handleNumber] = to;
                      }
                  });
              }
  
              // If using one handle, check backward AND forward
              else {
                  b = f = [true];
              }
  
              var state = false;
  
              // Step 2: Try to set the handles with the found percentage
              handleNumbers.forEach(function(handleNumber, o) {
                  state = setHandle(handleNumber, locations[handleNumber] + proposal, b[o], f[o]) || state;
              });
  
              // Step 3: If a handle moved, fire events
              if ( state ) {
                  handleNumbers.forEach(function(handleNumber){
                      fireEvent('update', handleNumber);
                      fireEvent('slide', handleNumber);
                  });
              }
          }
docs: Moves handle(s) by a percentage (bool, % to move, [% where handle started, ...], [index in scope_Handles, ...])