In Monty Python and the Holy Grail, the Holy Hand Grenade is described as a “sacred relic” carried by Cistercian monk Brother Maynard (Eric Idle) and is used near the film’s conclusion to destroy a killer rabbit that blocks the path of King Arthur and his hapless Knights Errant. Although unusually ornate in design, the Holy Hand Grenade functions like any other hand grenade.
Particularly important is the part of counting to three after the pulling of the triggering pin (the surmounted cross), complicated by King Arthur's mental block on counting. The instructions for its use can be found in the fictitious Book of Armaments, Chapter 2, verses 9-21, parodying the King James Bible and the Athanasian Creed
…And Saint Attila raised the hand grenade up on high, saying, “O LORD, bless this Thy hand grenade that with it Thou mayest blow Thine enemies to tiny bits, in Thy mercy.” And the LORD did grin and the people did feast upon the lambs and sloths and carp and anchovies and orangutans and breakfast cereals, and fruit bats and large chu… [At this point, the friar is urged by Brother Maynard to “skip a bit, brother”]… And the LORD spake, saying, “First shalt thou take out the Holy Pin, then shalt thou count to three, no more, no less. Three shall be the number thou shalt count, and the number of the counting shall be three. Four shalt thou not count, neither count thou two, excepting that thou then proceed to three. Five is right out. Once the number three, being the third number, be reached, then lobbest thou thy Holy Hand Grenade of Antioch towards thy foe, who being naughty in My sight, shall snuff it.”
Arthur then holds up the Holy Hand Grenade and cries out “ONE! TWO! FIVE!” Sir Galahad corrects him, shouting “Three, sir!”. Arthur then yells "THREE!" and hurls the grenade at the killer rabbit. The Grenade soars through the air, accompanied by a short bit of choral music, then bounces once and explodes. The killer rabbit dies in the explosion, and the Knights subsequently enter the cave that it had been guarding.