Let's know about GOOGLE DANCE ..

What is the GOOGLE DANCE ? 

The name "Google Dance" is commonly used to describe the index update of the Google search engine. Google's index update occurs on an average of once per month. So, It can be identified by important movement in search results and particularly by Google's cache of all indexed pages reflecting the status of Google's last spidering. however the update doesn't proceed as a switch from one index to a different at one point in time. In fact, it takes many days to complete the index update. throughout this period, the old and the new index alternate on http://google.com. At an early stage, the results from the new index occur periodically. however later on, they seem additional more. Google dances.

Trigger Factors of Google Dance

• Modification of text in images to HTML
• Social interaction got increase.
• Abnormally Increased in backlinks (specially high PR )
• Making the website more responsive 
• Making the website more faster (CSS &Java mimification, lazy loading, more faster servers, ..)
• Changing hosted IP address of website (sometimes the Google might be penalizing an IP address cause of changing)
• Competitors are being penalized for SEO type black hat SEO

How to Know Google Dance or Penalty?

Unfortunately, there's a No way of identifyin any of these cases.
In fact if you improve your User-interactions like easy interface, fast speed, more responsive or adding values for user you might be experiencing Google Dance.Blackhat expert Steve -" The google dance is a what GreatPVA's said and the indexing of your backlinks combinedly. It makes your rank up and down pages, normally lower down a few ranks, then jumps up higher than what you were on rank before. If you are indexed a lot of backlinks or sent a lot of them to your money website and then stopped or it is a newly registerd domain, it will either get the indexed or a penalty. Google dance's is a usually good thing. At least, from my experience".