Partial collar strikes occur when there is a malfunction of the striking press. This causes the collar to be in an incorrect position. The lower die (usually the reverse die) is recessed in the collar. This allows the coin which is going to be struck to have a formed rim. After a coin is struck the lower die raises upwards, pushing the struck coin out of the collar and ejecting it.
If a blank entering the collar is not properly seated, it will only have partial reeding as it is struck. The edge of this coin will have a partial reeding and a partial blank surface area. Recently, the Mint has installed new machinery where either die can be installed in either position.