The thing is that if you have a capacitor with 5V across it, like when the DTR is zero and the reset is pulled up to 5V and you take that end that has a zero and make it 5V, in that instant the end that was 5V now becomes 10V because the charge on the capacitor can not change instantly. This is the basis of of voltage doublers.
The diode shorts out that 10V to the 5V rail and so quickly discharges the capacitor.
The diode shorts out that 10V to the 5V rail and so quickly discharges the capacitor.
