code: function relaxLeftAndRight (alpha, id) {
          var columnsLength = columns.length
  
          columns.forEach(function (nodes) {
            var n = nodes.length
            var depth = nodes[0].depth
  
            nodes.forEach(function (node) {
              // check the node is not an orphan
              var nodeHeight
              if (node.sourceLinks.length || node.targetLinks.length) {
                if (node.partOfCycle && numberOfNonSelfLinkingCycles(node, id) > 0) {
  
                  /* empty */
                } else if (depth == 0 && n == 1) {
                  nodeHeight = node.y1 - node.y0
  
                  node.y0 = y1 / 2 - nodeHeight / 2
                  node.y1 = y1 / 2 + nodeHeight / 2
                } else if (depth == columnsLength - 1 && n == 1) {
                  nodeHeight = node.y1 - node.y0
  
                  node.y0 = y1 / 2 - nodeHeight / 2
                  node.y1 = y1 / 2 + nodeHeight / 2
                } else {
                  var avg = 0
  
                  var avgTargetY = mean(
                    node.sourceLinks,
                    linkTargetCenter
                  )
                  var avgSourceY = mean(
                    node.targetLinks,
                    linkSourceCenter
                  )
  
                  if (avgTargetY && avgSourceY) {
                    avg = (avgTargetY + avgSourceY) / 2
                  } else {
                    avg = avgTargetY || avgSourceY
                  }
  
                  var dy = (avg - nodeCenter(node)) * alpha
                  // positive if it node needs to move down
                  node.y0 += dy
                  node.y1 += dy
                }
              }
            })
          })
        }
docs: For each node in each column, check the node's vertical position in relation to its targets and sources vertical position and shift up/down to be closer to the vertical middle of those targets and sources