The trysail provides enough thrust to maintain control of the ship (e.g. 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.
Sleight, Steve (1999). The Complete Sailing Manual. ISBN 0-7894-4606-5,
Sails spars and rigging