Normally, horses only have a set amount of dashes you can perform before it has to regenerate.
By using the jump off animation and immediately remounting the dashes get refreshed. This basically allows to keep dashing without breaks and speeds up the average horse travelling speed by a lot.
You will need to have a shield, bow and arrows equipped for it to work.
To do it, hold the inputs for Shield Sliding (standard ZL + A), then hold ZR and press X. Your dashes will be refreshed.