Now that I think about it, I don't think that is called a dip needle, more like a Miner's compass. The Spanish dip needles are like two pairs of knitting needles loosely tied together at the ends and two people use then sort of like a "Y" rod or forked stick (actually like 2 "Y" rods connected) that is used for water dowsing. Like the Ouija Board cursor, if one person flinches the other responds. That has to be extremely difficult to use without getting false responses, but never used one.
Of course if you do an internet search you end up with the Miner's Compass. I don't know the percentage of inaccurate info on the net but it's huge. The dip needles pretty much a lost art. So lost, most people do not even know what they are and confuse them with the Miner's Compass. Fred Stewart used to sell them. The photo shows the needles angled upwards and the info says two people hold them facing each other and to follow the direction they lean towards, then when over the target them pull down or dip.
And I've never prospected with a Miner's Compass and never used the Spanish Dip Needles, either. Just thinking about it with two people it might be that one person goes into a trance and the other sort of guides them around. So when they get a hit, the other person magnifies the response. So it could be very sensitive in the right hands, but looks horribly difficult to me--sort of like a walking Ouija Board. LOL This is all just a guess.