The shalshelet trope note appears in 4 places in the Torah:
Breishit 19:16, Bresihit 24:12, Breishit 39:8, and Vayikra 8:23.
In all cases the note is on the 1st word of its verse and is followed by a psik. I understand that part of the trope notes function is grammatical. I'm assuming that there is a grammatical reason that requires shalshalet to be on the 1st word and must be followed by a psik. Why?