There is a whole spectrum of steps to enforce this. And it doesn't matter if your income goes down after you pledge--you can't simply state "I pledge [amount] on the condition that my financial situation remains stable". Do that, and joke-hova will insist that you pledge it no matter what, and then that thing will devastate you financially and force you to keep the pledge anyways. And blame Satan.
First round, it will be at the congregation level. If the whole congregation dogs it, the hounder-hounder goes after the hounders. They will start going after the flock, ensuring the donations come in under threat that it displeases that beady-eyed monster joke-hova if people don't give what they pledge. Guilt trips from the platform come at this point.
Next level, they will start approaching individuals. They start requiring each member to donate, giving them a way to track who donates what. Those who fall short can be hounded. This can range from simple guilt trips to being denied "privileges(??)", to being marked (I thought that was only for freeloaders and those who have to stick their noses into everyone's business), and ultimately disfellowshipping. They could use the "brazen conduct" card, or they could simply state that if you pledge and do not give that amount, the pledge is a lie and you would be a liar.
At the next level, they could treat these pledges as signed, legally binding contracts. Your very baptism is itself a contract and the baptism its signature, and it takes one scumbag judge to rule it as such. At this point, they could sue for the pledge arrears plus court costs, and added fees and time wasted going after you. This could also be used to enforce pious-sneer quotas, and to get people to pious-sneer or even go after those who refuse to do anything at all. It just takes one court to uphold either the pledge, your baptism, or both as legally binding contracts, and they could garnish your wages or deduct from your bank account whatever you pledge plus what they feel are "reasonable" fees.
Maybe now is the time to start getting your bank account pared down and invest in silver (or gold) which you keep?