Well, reserves doesn't really apply- that's a 40K term. Your best bet is to find a BoC army book and look up the actual rule.
What happens is, if you have any units that can ambush, one half of them do not have to be deployed at the start of the game. (If he only has one ambushing unit, it cannot ambush.)
At the start of his second turn, or at any turn thereafter, he may decide to have his ambushing unit come on the table. At the start of his remaining moves phase, he would place a marker for each unit at the point on the table edge he wants that unit to enter, and roll for its leadership.
If he rolls above the leadership, a scatter die is rolled in the center of the table, and the unit enters play from the point on the table edge indicated by the arrow.
If he rolls the leadership or below, the unit comes on at the marked point. Every model in the unit moves on from that point, and cannot march (or charge).
If I recall correctly, no mortal general has the ambush rule, so he won't be able to use this ability at all.
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