Porson's Law states that, if the third anceps (i.e. the bolded x above) is long and followed by a word break, then it must be a monosyllable.
A simpler summary of the Law is provided in W. W. Goodwin's Greek Grammar: M. L. West states it slightly differently, to take account of a rare situation not accounted for by Porson, where the word-break is followed rather than preceded by a monosyllable (e.g. Euripides, Heraclidae 529): Some examples of normal tragic trimeters which do not break Porson's Law are the following from Sophocles' Oedipus Tyrannus: In tragedy, as West observes, there are very few breaches of Porson's Law.
When the manuscript tradition, therefore, transmits a line that breaches Porson's Law, this is taken as a reason for suspecting that it may be corrupt.
[7] Similar laws which have been discovered in the dactylic hexameter are that if a word ends the fifth or fourth foot it is almost never, or only rarely, a spondee (– –).
The philologist W. Sidney Allen suggested an explanation for all these laws in that it is possible that the last long syllable in any Greek word had a slight stress; if so, then to put a stress on the first element of the last iambic metron, or the second element of the 4th or fifth dactylic foot in a hexameter, would create an undesirable conflict of ictus and accent near the end of the line.