The manner in which events are handled in the Matlab ODE Suite is discussed by C.B. Moler in his article "Are we there yet? - Zero crossing and event handling for differential equations" which appeared in Matlab News & Notes, (1997) pp. 16-17. The same procedures are employed in dde23. Various event location issues which affected the development of dde23 are discussed by L.F. Shampine and S. Thompson in the paper "Event location for ordinary differential equations" which is to appear in Computers & Mathematics with Applications. You can click on the following links to read these articles.