Jumping can look pretty scary but not to worry, the people jumping high fences have a lot of practice. Most horses approach a fence going around 12 ft./s (1 stride). To calculate when a rider should take off when jumping a 5.5 ft. fence will be shown below:
The average angle of takeoff=40°-45° (in our example we will use 40°) 
To find x we must use SOHCATOA:
Sin θ= O/H Cos θ= A/H Tan θ=O/A
Because of the information we know we will use Tan θ=Opposite/Adjacent:
Tan 40°= 5.5 ft./X
1x 5.5/tan 40°=X
Tan 40°=.839 ft.
1x 5.5ft/.839 ft.= 6.55 ft.
Now we know that the horse should take off 6.55 feet before the jump to keep its consistent pace. For the rider this means when counting strides you should take off in the middle of your count. This means when going through a line (Jump A to Jump B) that is 70 feet (5 strides) in the middle of the 5th stride the rider should release the reigns and let the horse jump.
:Stinner, Art. “The Physics of Equestrian Show Jumping.” The Physics Teacher25 (2014): n. pag. Web. 5 Mar. 2016.