A Vertical Fingertip Thrust (also called a Straight Fingertip Thrust, Spearhand Strike, Spearhand Thrust, or pyonsonkeut sewotzireugi) is a Spearhand Thrust that's performed with the hand held vertically (rather than horizontally).
The thrusting hand is chambered down at the waist, with the hand already vertical. In other words, the palm of your hand should be against your waist. From there, the hand is thrust forward as with a punch, the hand held vertical throughout the movement.
In the forms Taegeuk Sa Jang and Do-San for example, the Vertical Fingertip Thrust is supported at the elbow by a knifehand. The idea is that the off-hand has "swatted away" an incoming strike by swatting the knifehand downward, and now a spearhand is coming in above the knifehand to strike.