The "cross" drill is a variation of the inside-out, side-to-side and advance retreat. In it's purest form, it combines the drills together in that the player is made to move side-to-side, inside-out, and advance retreat. The concept is that that player is being pushed into uncomfortable positions - similar to something that the opponent may throw her way - and have to generate pace and precision accordingly.
In other words, this drill borrows from the muscle-confusion theory of physical workouts (such as P-90X or Insanity) by making the player execute shots when she is not in the optimum location. For example, the player is made to hit a "regular" forehand, followed by an inside-out forehand, followed by a short forehand which is, in turn, followed by a retreating forehand. From then, the player is jerked around from these four points of an imaginary cross while always moving either after the ball or away from the ball. In this drill, the player should imagine that s/he has a Death Star "force-field" around her and that the ball never enters the force-field (thereby jamming the player). The key is for the player to "tell the ball what to do"; not the other way around. For maximum benefit, alternate a the cross drill with a live ball rallying drill where the coach pushes the player gently (at first) around the court making him move backwards, sideways and forwards, and seeing how the player adjusts to the ball and recovers after hitting. The emphasis should be on proper footwork, balance, contact point, intensity as well as hips-shoulder-arm preparation and positioning.