Apple Shake 4 - Manual (Page 319)

Chapter 10 Parameter Animation and the Curve Editor 319
Linear Splines
Linear(cycle,value@key1,value@key2,...value@keyN)
LinearV(time_value, cycle,value@key1,value@key2,...value@keyN)
With Linear splines, there is not much mystery. No smoothness, but you know exactly
what you get.
Step Splines
Step(cycle,value@key1,value@key2,...value@keyN)
StepV(time_value, cycle,value@key1,value@key2,...value@keyN)
Step splines create a stair-stepping spline that maintains its value until the next
keyframe. This is great for toggling functions.