The story
Hiraṇyakaśipu had a boon: not to be killed by man or beast, indoors or out, by day or night, on the ground or in the air, by any weapon. He then forbade the worship of Viṣṇu, and his own son Prahlāda kept it anyway.
Asked whether his god was in the pillar of the hall, Prahlāda said yes. His father struck the pillar and Narasiṃha came out of it — neither man nor beast, on the threshold, at dusk, on his own lap, with claws.
The story is read as an argument about the letter and the spirit of a promise, and Narasiṃha is invoked as the protector of those who are cornered.