Ok so there is a hill midfield and I ambush a unit behind it. The opponent for whatever reason gets the drop and advances on the hill before I can move to the top. So can I 'reveal' myself and stop his movement as he approaches before he climbs the hill or can I not stop him from keeping going until he reaches the crest and sees me. Cheers.
Ambushes are revealed when the enemy UG could see them (p181). As visibility over a crest is 2BW (Terrain Table), the enemy UG stops 2BW away from your ambush. This means that the enemy UG will stop short of the crest (assuming the crest is on your side of the centre line).
If the crest is on your opponents side of the centre line you would be visible from their half of the table so could not ambush.
Richard