1)
What is the meaning of "b'Chafzam"?
1.
Rashi: It is in their haste [to flee].
2)
Why is it written b'Heichafzam, and we pronounce it b'Chafzam?
1.
Radak: It is written with a Hei, i.e. Nif'al. It is pronounced without a Hei, a simple conjugation.
