From the manual:
"Turbo Ramp Rate: This is the literal ramp rate of whatever your turbo timing is set.
A turbo setting of 20 will give you the following reaction:
1.0 Ramp rate = 20 degrees in 1 second applied
2.0 Ramp rate = 20 degrees in .5 seconds applied
3.0 Ramp rate = 20 degrees in .30 seconds applied"
--> It is the the 'punch/curve' setting for turbo.