Heroin is often injected as a liquid to give an instant 'rush' - hence the tying of a tourniquet to restrict the blood flow to the arm, and make a vein rise to make injection easier. When the tourniquet is released, the heroin solution quickly spreads through the bloodstream to the brain, for the ecstatic rush that early use provides.
Unfortunately, the drug is highly addictive, and addicts need more and more stronger doses, not to feel the rush, but simply to avoid feeling like death. Withdrawal involves dreadful muscle pain, cramps, hallucinations, and is seriously difficult to achieve.
The answer - leave it alone, it's a silly person's 'fun'.