This is how it goes, at oneuptrader at least. the numbers may vary slightly based on the company. Some do not have this trailing drawdown, I think maybe most do, but they have some variation of this.
Day 1: $50k balance, Drawdown $47500. Profit 1k, Drawdown goes up 1k.
Day 2: $51k balance, Drawdown now $48500. Lose $500. Drawdown doesn't move.
Day 3: $50500 balance, Drawdown still $48500. Profit 2k. Total profit is +$2.5k, so drawdown moves up $1.5k to equal $2.5k above initial of $47500 = $50k.
Day 4: $52500 balance, Drawdown $50k. Lose $1k. Drawdown doesn't move.
Day 5: $51500 balance, Drawdown still $50k. Make $5k.
Day 6: $56500 balance, Drawdown stays $50k forever, until you go below $50k (you shouldn't anyway but that's the only way you fail).
Get it?
However, when you withdraw your $3.5k one day and your account goes down to $53000, you have to be aware your drawdown is still $50k. It will remain there for life.
For what it's worth Earn2trade has the same system. I haven't looked at all of them in depth yet but I will soon.
"This continues until your trailing drawdown reaches the starting balance of your account, or in this case $25,000.00, at which point the drawdown will no longer trail above $25,000.00."