Everyone who powers up puts 5% into a pool that is given out daily to HP holders. Only 2% of the total pool is given out daily. So as people cash out those who remain are still getting that 2% of the pool from everyone who put in 5%. So over time you would make the 5% back + more depending how long you stayed powered up. Its not really a tax per say as it goes right back in your pocket unless you cash out quickly.