Step 1: Have a lot of USD and a lot of BTC - market price at $13.90
Step 2: Place an ask order for $150k BTC at $14.25 - build an ask wall
Step 3: Sell $100k worth of BTC - break the bid wall
Step 4: Hope for a sell-off
Step 5: buy them back cheaper!
Pretty clear there's some big players now (as opposed to the early days). It now takes $100k to move the market (used to take $10k).
When the market is bearish and thin, its easier to manipulate down (spark panic selling). When the market is thin and bullish, easier to manipulate up (spark panic buying).