Day traders generally use margin leverage; in the United States, Regulation T permits an initial maximum leverage of 2:1, but many brokers will permit 4:1 leverage as long as the leverage is reduced to 2:1 or less by the end of the trading day. In the United States, people who make more than 4 day trades per week are termed pattern day traders and are required to maintain $25,000 in equity in their accounts. Since margin interest is typically only charged on overnight balances, the trader may pay no interest fees for the margin benefit, though still running the risk of a margin call. Margin interest rates are usually based on the broker's call.
The first type of scalping is referred to as "market making," whereby a scalper tries to capitalize on the spread by simultaneously posting a bid and an offer for a specific stock. Obviously, this strategy can succeed only on mostly immobile stocks that trade big volumes without any real price changes. This kind of scalping is immensely hard to do successfully, as a trader must compete with market makers for the shares on both bids and offers. Also, the profit is so small that any stock movement against the trader's position warrants a loss exceeding his or her original profit target.
Ultimately, each swing trader devises a plan and strategy that gives them an edge over many trades. This involves looking for trade setups that tend to lead to predictable movements in the asset's price. This isn't easy, and no strategy or setup works every time. With a favorable risk/reward, winning every time isn't required. The more favorable the risk/reward of a trading strategy, the fewer times it needs to win in order to produce an overall profit over many trades.