Anti addition of hydrogen atoms to the the triple bond occurs when alkynes are reduced with sodium (or lithium) metal in ammonia, ethylamine, or alcohol at low temperatures. This reaction called, a dissolving metal reduction, produces an $$(E)$$- or $$trans$$-alkene.

Sodium in liq. $$N{H_3}$$ is used as a source of electrons in the reduction of an alkyne to a $$trans$$ alkene.