How does supF mutation suppress an amber mutation?

The amber mutation is change of an amino acid codon to a stop codon (UAG). SupF is a mutant tRNA that recognizes UAG but is charged with Tyr, so it places a Tyr where the mRNA should stop translation. There are lots of wt tRNAs in the cell that do put in a proper stop codon, but the functional gene product resulting from supF rescue will be dominant over the loss-of-function RNAs (typical hypomorphic inheritance). Transcription from the allele will not increase to maximize functional protein levels, unless there is a positive feedback loop.