Simone, do you mean that you want to extract those regression coefficients associated with the estimate HDR curve? If so, you can use something like
3dbucket -prefix myHDR myInput+tlrc'[a..b(c)]'
where 'a' and 'b' are the beginning and end sub-brick index while 'c' is the gap between two consecutive regression coefficients (e.g., 2 or 3). Then you can load the file myHDR+tlrc into AFNI and use the 'graph' button to visualize the HDR shape.
Gang