Via the FT:

Airlines that offer this either send travellers an email inviting them to upgrade or travellers can go direct to the airline’s website.Once you’re on their site, rather than them saying, ‘you can pay £400 to upgrade’, you can now say, ‘I’ll pay £300’,” explains Ken Harris chief executive of Plusgrade, the technology company behind the platform. “There may be a minimum upgrade price and there will be an indicator which shows the strength of your offer.” The airline will then email you at least 72 hours before your flight to tell you if your bid has been successful; if not, you retain your original reservation.

Taking things to another comfort level:

Along with upgrades, you can also pay a nominal fee to ensure the seat next to you is not occupied.

To extract surplus and reduce information rents from high types, “low” types must be punished with inefficient allocations. Expect to be seated en masse if you do not purchase an upgrade even if the plane is half empty.