Specifically, he will threaten to disallow wire transfer of money from US to Mexico by non-citizens. Since that is a revenue source they greatly depend on, he suggests they'd rather fund the building of the wall. If they agree to fund the wall, then the ban on transfers will go away.
https://assets.donaldjtrump.com/Pay_for_the_Wall.pdf
Specifically, he will threaten to disallow wire transfer of money from US to Mexico by non-citizens. Since that is a revenue source they greatly depend on, he suggests they'd rather fund the building of the wall. If they agree to fund the wall, then the ban on transfers will go away.