As the name "hd_idle" says, I assume that a spindown command is only issued when the disk is idle. Anything else doesn´t make any sense. In the readme of hd_idle one can read "hd-idle is a utility program for spinning-down external disks after a period of idle time". hd-idle utility, modified to use hdparm tool to spin down disks
I just went with the brief description on the github page that said "hd-idle utility, modified to use hdparm tool to spin down disks" I guess I should RTFM. :-). Thanks for the clarifications.