Trysail

A trysail (also known as a "spencer") is small triangular or square fore-and-aft rigged sail hoisted in place of a larger sail when winds are very high.

The trysail provides enough thrust to maintain control of the ship (eg to avoid ship damage, and to keep the bow to the wind). It is hoisted abaft (i.e., directly behind) the mainmast (taking the place of the much larger mainsail) or, on a brig, abaft the foremast.