>>59391949The description of its effect differs between F/SN (Saber vs Gil at the very end - just the wind torrent compressed by folding dimensions), F/Z (the landscape was turned into a hellish zone, with violence comparable to "splitting heaven and hell apart") and F/HA (similar, but he also opened a "gateway to hell", which is pretty much equivalent getting shoved into Earth's core - good luck surviving that).
Also, in the first one, he very clearly contols the weapon output even without the true nature revealed - first a long but weak burst Saber is able to withstand with just the prana barrier and Avalon's regeneration, then increasing the power once he realises she won't give up. Heck, she needed to fire a low-powered Excaliblast in order to create a gap large enough to allow her to activate Avalon and survive the leftovers.
http://www.youtube.com/watch?v=dJMXWx6l1eQ&t=4m2s - here's that part, along with continuation in the next video