How does the Massachusetts Personal Liberty Act of 1855 defy the Fugitive Slave Act of 1850?
The Act of which you speak was one of several "Personal Liberty Laws" passed in northern states in response to the Fugitive Slave Act, part of the Compromise of 1850. The Fugitive Slave Act placed runaway slaves under federal jurisdiction and was heavily weighted against runaways. It was manifestly unfair, so much so that there was real danger of free blacks being arrested as potential runaways and given little opportunity to defend themselves. The Act caused tremendous uproar in the North. Ralph Waldo Emerson commented:
the filthy enactment was made in the nineteenth century by people who could read and write
Personal Liberty Laws were attempts by northern states to prohibit local officials from enforcing the law. Ironically, their position--that the Act was unconstitutional and thereby unenforceable within the state's borders--bore a striking resemblance to John C. Calhoun's doctrine of Nullification when Calhoun, a southerner, opposed the Tariff of 1828 which had benefited the North.
Personal Liberty Laws took several steps to contravene the Fugitive Slave Act. Some forbade state and local officials from enforcing the Act; others prohibited the use of state or local jails for incarcerating suspected runaways; others placed the burden of proof on the slave catcher, this being a direct contradiction of the Act itself.
The U.S. Supreme Court declared Personal Liberty Laws unconstitutional in the case of Prigg vs. Pennsylvania. The Court held that states could not interfere with the enforcement of a Federal statute although they were not compelled to enforce it.
The Massachusetts Personal Liberty Act prohibited law enforcement officers in Massachusetts from enforcing the Fugitive Slave Act, a federal statute. The Fugitive Slave Act was unfair on its face, as constables and magistrates were paid a higher fee for certifying that a prisoner was indeed a fugitive than if they determined that he was lawfully free. Accused persons were denied jury trials and could not call witnesses in their own behalf. This was all part of the Compromise of 1850 which saw California admitted into the Union.
Unfair is it might be, the Fugitive Slave Law was a federal statute, which under Article VI Clause 2 of the Constitution superseded any state law:
This Constitution, and the Laws of the United States which shall be made in Pursuance thereof; and all Treaties made, or which shall be made, under the Authority of the United States, shall be the supreme Law of the Land; and the Judges in every State shall be bound thereby, any Thing in the Constitution or Laws of any State to the Contrary notwithstanding.
The Personal Liberty Act was in direct contravention of the Constitution; something one would have expected from nullifiers such as John C. Calhoun. The fact that a Northern state openly defied a federal law is ironic.