You number the candidates for your constituencies seat from most preferred to least.
Then when they're counted up, a candidate needs 50% of votes. So you look at all the 1st preferences, if no one has 50%, you drop the person in last place, and assign their 2nd preferences around the other candidates, if no one has 50% yet, you repeat the process and so on and so on until someone has 50%.
In use, it will make very little difference other than getting the Lib Dems a few extra MPs in English swing-seats.