How do some people eject a thin projectile stream of saliva from under their tongue? and what is this called? can everyone do it?

Salivation: It is the act of salivating from the sub-mandibular duct. The action is an involuntary. Very common and normal physiological act.

Saliva stream: The ducts from the major salivary glands under the tongue meet in a small midline nodule ind allow saliva to enter the oral cavity. Some people can case rapid constriction of this nodule, causing a stream of saliva to be expressed. It's called a squirting caruncular in that case. Like being able to wiggle your ears, only a small minority of people can do this.