As tensions in Ukraine continue to escalate, the White House says that U.S. sanctions have had a "significant impact" on the Russian economy. But the real effect on President Vladimir Putin's strategy has been hard to see.

However, one person who has likely seen a boom in business from the latest round of sanctions is British lawyer and barrister Maya Lester.

Lester fights for the rule of law by defending the very people targeted by international sanctions. Most recently she's been getting calls from Russians that have been sanctioned over the Ukraine crises. But she's made a successful career helping Iranian, Syrian, Egyptian, and Burmese clients get off targeted blacklists over the past two decades.

Lester says she fights for her clients without spending a whole lot of time worrying about who they are. Often times, she says her clients aren't bad guys—and that it's her job to help fight for their names to be removed from these lists.

"Sometimes we're not in the business of saying, 'This is a wonderful person and he shouldn't be listed,' although very often that is the case," she say. "It's usually a question of saying that what has been put forward by European decision makers is either incorrect or it's insufficient to justify including someone on the list."

Lester says that in most cases, sanctioning countries feel that it's not appropriate for courts to interfere in international political decisions. She does, however, have some legal recourse to challenge sanctions against individuals.

"The court will be less interested in the question of, 'Should we have sanctions against Iran, Zimbabwe or Russia?" she says. "The question they are interested in is, 'Has due process been afforded to this particular individual?' That usually comes down to if the person has put in evidence that what has been said about them to justify their listing is wrong."

Lester says that in many cases, a court is often interested in what the sanction decision maker has to say about the evidence presented to justify an incorrect listing, adding that the court will consider additional evidence from the sanction decision maker and determine if the reasons given for blacklisting someone are clear and comprehensible.

However, the process can be much more nuanced depending on which government is handing down the sanctions.

"People talk about international sanctions, but usually the decision makers that impose sanctions are either the U.S. or the European Union or sometimes the United Nations," she says. "The particular process that's followed, and the way in which it's possible to challenge sanctioning, really depends which of these regimes we are talking about. But each of them has their own rules of due process."

Lester says that there is no advanced noticed when an individual is blacklisted.

"The purpose of these asset freezing measures is to freeze your assets—if you're told in advance that you might be listed, there is a risk that of course you will dissipate your assets," she says.

While individuals do obtain notification after they are blacklisted, that hasn't always been the case. Lester says that after 9/11, the United Nations placed large numbers of people on its Al-Qaeda/Taliban blacklist at the request of the United States—the U.N. did not provide any notification to the individuals, or reasoning for why they had been placed there in the first place.

"Now what happens in the European Union and I think in the U.S., you are informed either by a letter or by some kind of public notice that you have been added to the list, and you should be told why," she says. "The questions that the courts have been struggling with, particularly in the European Union, is what kind of reasoning is sufficient, how detailed do they have to be, and do they have to be supported by evidence."

The extent of what an individual on a blacklist can and cannot do varies nation to nation and by specific sanction. If a person were currently facing Russian sanctions, for example, that individual would be barred from having funds or any kind of economic resource made available to them in the European Union. Additionally, all of that individual's existing funds within the European Union would be frozen.

"It would be a criminal offense for anyone to make funds available to you in the European Union," says Lester. "If you are someone that's been blacklisted, the restrictions are on what you can do when you're in the European Union, or what E.U. citizens can provide you with."

Family members of sanctioned individuals are not automatically blacklisted, but those individuals also face certain restrictions.

"Certainly, someone's spouse would not be able to go to the European Union and make funds available in London for the benefit of someone on one of those list without being in danger of committing a crime," she says. 

When it comes to business enterprises, there has been limited blacklisting of companies, but these entities can still grapple with challenges that come as a result of sanctions.

"The difficultly comes with dealing with companies where people who are on the list have a large interest in the company," says Lester. "The E.U. can add either a bank or a company to the list, and if they do that then that company is under the same restrictions for people. But there is also now a great danger of dealing with companies where they haven't themselves been listed, but say there is an oligarch on the list that has a great degree of ownership or control of the company, then again they need to do their due diligence and find out whether a Russian company that they are dealing with is owned or controlled by someone on the list. If they are, than sanctions with also bite to prevent people from dealing with companies."

If the U.S. or E.U. were to place sanctions on a nation, Lester says that is common for the blacklisted country to find other ways to preserve trade by finding alternative avenues.

"It's quite natural that what that country will try and do is make sure that its trade is not disturbed," she says. "I have heard that China is a beneficiary of some of the sanctions that the European Union has imposed. A country subject to sanctions will say, 'Fine, we can't easily do business with the E.U., we'll do business with the Far East instead.' I think it's less a question of particular banking or tax havens, and more the flows of trade that will naturally be effected by sanctions."

So do sanctions really work? Well, that depends on the goals that a nation is trying to achieve, says Lester. Asset freezes and travel bans are usually used as a foreign policy tool to try and avoid putting troops on the ground.

"But what they're trying to achieve is so disparate," she says. "If you think about stopping the funding of terrorism, or trying to persuade a country to abide by the rule of law, or trying to persuade President Putin to act differently in Ukraine, or trying to stop nuclear proliferation, or even returning assets to Egypt—these are such different foreign policy goals, that it's extremely difficult to measure whether sanctions are working or not working. I'm really more concerned with when sanctions are targeted at individuals, whether they do so in a fair way, rather than whether they achieve their goal."