code: public function onBlockToHtml(Varien_Event_Observer $observer)
      {
          if (!$this->canCollect()) {
              return;
          }
  
          /* @var $block Mage_Core_Block_Abstract */
          $block = $observer->getData('block');
  
          // Last chance before rendering toolbar to fetch updates (queries triggered from blocks)
          if ($block->getNameInLayout() == 'debug_panels') {
              $this->updateProfiling();
          }
  
          if (!$this->canCaptureBlock($block)) {
              return;
          }
  
          $blockName = Mage::helper('sheep_debug')->getBlockName($block);
  
          $requestInfo = $this->getRequestInfo();
  
          try {
              $blockInfo = $requestInfo->getBlock($blockName);
          } catch (Exception $e) {
              // block was not found - lets add it now
              $blockInfo = $requestInfo->addBlock($block);
          }
  
          $blockInfo->startRendering($block);
      }
docs: Listens to core_block_abstract_to_html_before event and records blocks that are about to be rendered.
  
  @param Varien_Event_Observer $observer