How did the two DNA magicians do their card trick on Britain’s got talent 2017 semi final?

The DNA magicians impressed once again with their card trick and the word ‘probably’ on Britain’s got talent 2017 semi final and left Simon Cowell asking, how did they do that?

The duo asked David to think of a word in the English Dictionary, they then asked him to write the word down and put his signature on the piece of paper. They then asked each of the judges to pick one card from a deck of cards that had been mixed up. Alesha then had to decide which judge got which card from the three that had been picked. The duo managed to guess which card each judge had from a random deck.