Not sure how this scheme will work as it is only a trial.
But in principal, you sell back you ticket for 75% of pro-rata of the Season Ticket price, so in affect still paying 25% of the reduced season ticket price. The club then sells it to another supporter for full price. That person who attends might buy a drink etc...enjoy it and then decide next year to buy a season ticket.
If say 1000 people given back their ticket, and they sell all 1000 tickets. The club will also know that there is demand to look at possibly extending the ground, selling more merchandise etc.