From the FAQ link on the team page:
Q: Can my assistant coach sign recruits for me?
A: Yes and no. If you have not made any recruiting efforts for the current recruiting period and have more than 3 scholarships open, the assistant coach will fill all but 2 of the open scholarships (maximum of 5 signings). This is by design to assist coaches who were out of town during recruiting and to address schools which have been abandoned.
As for the timing, I think the "assistant coach" works in similar fashion to the Sim AI second push of recruiting and takes place at the same period of time, late on the final day of signings.