the signal device is free to fall by {HEAD ~ gravity} into {TAIL ~ a vertical position} when triggered by opening the mailbox door 
into
the signal device is free to fall by { HEAD ~ gravity } into { TAIL ~ a vertical position } when triggered by opening the mailbox door
OTH OTH OTH OTH OTH OTH OTH OTH HEAD HEAD HEAD HEAD HEAD REL TAIL TAIL TAIL TAIL TAIL TAIL TAIL OTH OTH OTH OTH OTH OTH OTH