When the going turns tricky and the only cracks you can find are too narrow for conventional nuts, it’s time to pull out the micro wires. It’s always better to have something, however small, between you and the ground.
IMP stands for Immaculate Marginal Protection – a neat little acronym, but one which fits perfectly.
IMP runners can never be considered as ‘trainstoppers’, at least not in the same way that a well placed Wallnut might be. But they can give unexpected holding power in marginal placements, particularly if you cluster the placements and equalise them so that the load is spread evenly.