RenTech soaks up arbitrage (the $20 bill laying around) and is able to compound this aggressively using leverage. Using a multi-strategy approach to arbitrage, the firm has room to 1) invest across a variety of different opportunities and 2) find new ones.
What a lot of people (especially day traders) don't understand is that there are a variety of situations where arbitrage exists.
For example, a merger announcement shoots the stock price up to "very near" the cash offer price. The difference is an arbitrage opportunity that, compounded, pays much more than the risk-free rate (you might get 3-4% annually buy making trades to eek out the final difference between the stock price post-announcement and the offer price). Occasionally, the spread between the target and acquirer widens (typically due to liquidity issues, or maybe a macro event), which allows arbitragers to scale into this position. This is just one example, btw, there are numerous forms of arbitrage that appear and evaporate.