Participating preferred = investor invests 50M on 1x participating preferred and company sells for 100M. The investor makes 50M + their % share of the company. Common stock makes % share of 50M.
They participate in the % sales ON TOP OF their preferred stocks liquidation preference (money in = money out before common stock value is considered)
Now here's where it gets nasty. 2x participating preferred = investor invests 50M and company sells for 100M. Investor makes 100M and common stock makes $0. Or if the company sold for 150M, investor makes 100M and participates in the % stock value from 50M.
It's a bit confusing, but the short is participating preferred is the absolute worst terms for common stock holders (founders included.)