Defence mechanism against strong (16+pts) artificial opening bids of 1♣ (as in Precision and Blue Club systems). The principle is to counter honour- point strength with distributional values.
An overcall shows a two-suited hand with at least 5-4 distribution in the suits pinpointed by the following bids over 1♣:
1♦ shows diamonds and hearts.
1♥ shows hearts and spades.
1♠ shows spades and clubs.
2♣ shows clubs and diamonds 1NT shows diamonds and spades Double shows clubs and hearts.
In fourth seat, after an artificial 1♣ opening and an artificial response (such as a 1♦ negative or a control -showing response), the principle is the same. The lowest four suit bids show the lower of touching suits; double shows non-touching suits including the suit doubled; no-trumps shows the other non-touching two-suiter.
You can also play Truscott over an Acol/Benjamin 2♣; all the bids are a level higher than over a strong 1♣.