A Sprint Smash is a simple technique not described anywhere in the game tutorials: you can immediately halt a sprint by using your melee weapon's basic smash attack (keyboard: click R). For example, in many boss fights where you have just dodged or run out of range of a boss smash, you can quickly get back into range with a Sprint Smash, then perform another short #2 combo (LR), effectively squeezing two smashes into a short boss vulnerability window. Or you can simply Sprint Smash > dodge away > Sprint Smash > dodge away > (repeat) to provide higher DPS against bosses with extremely short vulnerability windows.
Fiona characters can Heavy Cancel out of a Sprint Smash to recover even faster from the smash animation and sneak in an additional combo for higher potential DPS than other classes.
A Fiona will bash the enemy with her shield as her sprint smash, which will knock down mobs almost all the time but is low damage and will almost never stun a boss. An Evie will thrust out the hand holding the little energy ball she creates while sprinting and create a large blast. this can actually do good damage and is the only sprint smash with a skill: eagle talon, and it also comes with the rank 9 goodie of being able to spawn magic fire mines by holding down the button. A Lann will do a stab that is almot identical to his 0-hit smash, but slightly faster.