What is 'pig-butchering' crypto scam where fraudsters use romance to lure people?

Fraudsters are using the Chinese-origin 'pig-butchering' technique, a reference to how a target is "fattened up" before being butchered, for cryptocurrency scams. They develop long-term romantic relationships with targets on dating apps and social media to make them interested in cryptocurrency, forex and gold investments. They introduce the target to a sham investment website or app they control.