First, I did not think you could tie your ICR to an equity index like the S&P500 but had to tie it to a specific S&P500 index fund.
Second, as John stated, it is cumulative. The account balance goes up or down annually per the ICR and can and will be negative until such time as it gets paid out.
Assuming future pay credits of $10,000, a 12/31/2024 first year balance of $10,000 has negative interest of $2,000 after the 20% loss and gets $10,000 pay credit at 12/31/2025 for balance of $18,000. 10% gain in 2026 yields interest credit of $1,800 and $10,000 pay credit makes balance $29,800 at 12/31/2026, and so on. However, if this person left and was getting paid out, they would get $30,000 ($10,000 x 3 years) - unless the plan has defined ICR floor as may be permitted by the regulations, as John noted.