From Eterna Wiki

An example of the smurfing technique - note how bases 1 through 15 in the external loop are all uracil. Snapshot taken from a design by Eterna player honeyily.

Smurfing is a puzzle solving technique which involves painting most or all single bases in a puzzle blue (uracil) to prevent them from mispairing with something else. It can be seen in some of Wawan’s 2-2 and 1-2 puzzles and Tesla’s puzzlets. For smurfing tutorials, see Teaching and tutorial puzzles.