Question

How does CUT act as effective contraceptive for human females

Answer 1

actually it is Cu-T 
copper T or Cu-T is an effective contraceptive in following way:
1)It releases copper ion and these Cu ion released suppress sperm motility. 
2) this also decreases the fertilizing capacity of sperm.

Answer 2

Cu-T or copper-T is an intrauterine device used for birth control. Cu-T is used as an emergency contraceptive and acts as a long acting reversible contraception method.

The copper-T prevents pregnancy by action of Cu ions. The Cu ions inhibit the sperms from reaching the egg by restricting its movement. It does so by destroying the lipid architecture and biochemical structure of the sperm.

Even if a sperm fertilizes an egg, the Cu laden uterine environment prevents the fertilized egg from being implanted.