This block delays the Boolean input by an infinitesimal small time delay and therefore breaks algebraic loops. In a network of logical blocks, in every "closed connection loop" at least one logical block must have a delay, since algebraic systems of Boolean equations are not solvable.
The "Pre" block returns the value of the "input" signal from the last "event iteration". The "event iteration" stops, once both values are identical (u = pre(u)).