http://blogs.wsj.com/digits/2013/11/13/snapchat-spurned-3-bi...
http://allthingsd.com/20131029/snapchats-giant-funding-from-...
This is what it looks like to me:
Step 1) Buy 30% of a company at a $5 million valuation.
Step 2) Buy (or hopefully convince dumber investors to buy) 5% at a $3 billion valuation.
Step 3) Tell Yahoo to acquire you for at least $5 billion since your last raise was at $3 billion and you want your dumb investors to be happy.
Step 4) Cash your billion dollar check.