Grips

Slice (also Volley/Serve): index finger main knuckle is just below the upper slant
FH Rainbow (forehand): index finger main knuckle is on the lower slant, or just above it
2HBH Rainbow (2-handed backhand): index finger main knuckle directly on the slant
1HBH Rainbow (1-handed backhand): index finger main knuckle on the top

Grip Shout-Out
Have someone call out: Slice, Volley, Serve, Rainbow FH, Rainbow BH, and then change your grip by turning the racquet w/ your other hand.