The sprag is a one way clutch.
But this terminology is old, it is now an O/R clutch.
Or over run clutch.
A C4 for instance. A simplified description.
When the forward clutch or is applied the O/R clutch will hold and the result is First gear.
This is different from Low gear as the band is used there.
It is a good idea to replace the springs if you have a high mileage unit.
They will not hold if they're junk. Sometimes they'll slip, other times they wont. Sometimes they will break the outer race as well as the inner race, and sometimes its quite dramatic when that happens (case explosion). Luckily for us, C4s usually don't have much problem with the o/r clutch.