What is the effect of Progesterone-only pills?
Ref. KDT. Page. 343
Progestogen-only pills have multiple biological effects on the female body.
Multitude effect of POP:

Inhibition of follicular development
Inhibition of ovulation
Suppression of corpus luteum formation
Changes in the nature of the cervical mucus which inhibits the passage of sperms into the uterine cavity – Thickening of cervical mucous secretion
Effects on the endometrium which affect implantation – by making it thin
Completely suppresses ovulation
Thin lining of uterus
Thick cervical mucus
All of the above