..the account of the practice at Pudding Pie Hill in which a knife is stuck into the centre in order to hear fairy speech.. [is the something like the reverse of how a knife is used in] a story concerning Dun Borbe on South Harris, in the Hebrides. The fort was believed to be a fairy abode, and on one occasion:
a sailor of Harris.. sat down to rest on this fairy knoll and heard great lamenting therein. He was curious by nature and also kindly, so he set out to try to find out what was causing the Little People such distress. Being a practical man he decided that the best way to find out was to go into the dun and ask, and he set off walking round it slowly and carefully seeking the entrance. No sign of a door could be seen, but the cries and piteous sobbing continued, indeed seemed to grow more hopeless. He stood wondering what to do next, when he noticed a knife plunged to the hilt in the earth. Without thinking, he pulled it out; instantly an unseen door opened and out rushed the Little People, to surround him and, with cries of joy and welcome, to hurry him into the dun to their Queen. As soon as he saw here he asked what had been wrong. He felt very sorry for the Little People, who still showed signs of having been in great trouble; nevertheless, he wisely held fast to the knife while the Queen explained. She said that a man of the dun had loved a Harris maiden and they met and spent the long summer days together while she herded her father's cows. But her father had found out, and, being very angry, he had learnt from his daughter how to find the entrance to the dn and had then come and stuck his fisher's knife in the door frame, and they, unable to touch or pass cold iron, were prisoners in their dun, expecting to starve to death. He had saved them.
Erm I don't know what they gave him as reward. The story's from Otta F Swire's 'Outer Hebrides and their Legends' 1966, p77, and quoted in 'Circling as an Entrance to the Otherworld', by Samuel Pyeatt Menefee, in Folklore, Vol. 96, No. 1. (1985), pp. 3-20.