Dorsiflexion is when you pull your foot up (towards your shin). plantarflexion is obviously the opposite and is thus achieved when your calve muscles (gastrocs and soleus) are contracted.
If you do leg curls with your feet dorsiflexed you are using your hamstrings and your gastrocs, which is why you can move higher weights.
If you do them with your feet plantarflexed you are only using your hamstrings, as the gastrocs are already contracted in this position and thus ‘actively insufficient’ (can’t contract any further).
Which version is the better one for you therefore depends on what you want to achieve.
Leg curls are generally not very useful for sprinters, though, as you use your hamstrings in a closed kinematic chain and primarily as hip extensors when running while you use them in an open chain and as knee flexors during leg curls.
Regards,
Robin.