There is not measurement of a "market drop," only a decrease in the equities in the account - this includes BOTH the sale and the market loss.

Here is an example:

Month 0

STOCKS: $100

CASH: $0

Month 1

STOCKS $10

CASH $65

The account holder had their equity holding decrease by 90%. But that is composed of a market loss of $35 and their sale proceeds of $65.

65 / 90 = 72% (which is greater than 50% of the total decrease).

The 50% filter explicitly excludes those that just have market losses, the trader needs to have actively sold between 45-100% of their account value, combined with market losses between 0-45% for a total decrease of 90-100% between both factors.