sfplay~ wont reverse.

im making a sample/drum machine and a neccessity is start/end/pitch controls of the samples, among other things. and in the online reference of sfplay~, it says that a speed input of -1 will reverse the sample, and its not doing this. so im using the groove~ object to do this, which it does. but im not sure if its possible to use groove and have an end point control on it. so 2 questions arise.
1. why wont sfplay~ play the sample in reverse when theres a negative value for the pitch input.
2. is there a way to control a samples end point in groove~ without turning on looping.