Here is an example:
The first day, HGU up 20%, HGD down 20%
The second day, HGU down 24%, HGD up 24%
The price of HGU: A*(1.2*0.76)
The price of HGD: B*(0.8*1.24)
After x times of up and down:
The price of HGU: A*(1.2*0.76)^x=A*0.912^x
The price of HGD: B*(0.8*1.24)^x=B*0.992^x
You see both of them dropping :-)
The first day, HGU up 20%, HGD down 20%
The second day, HGU down 24%, HGD up 24%
The price of HGU: A*(1.2*0.76)
The price of HGD: B*(0.8*1.24)
After x times of up and down:
The price of HGU: A*(1.2*0.76)^x=A*0.912^x
The price of HGD: B*(0.8*1.24)^x=B*0.992^x
You see both of them dropping :-)