I think from a fence position, there are few attacks that can deliver so much damage as quickly as a straight palm to the face. One, sometimes two, of these following scenarios will happen...
1) You'll miss your target and strike their throat.
2) You'll miss your target and catching their chin with your palm, driving their head backwards.
3) You'll miss your target and drive your fingers into their eyes.
4) You'll miss your target and drive your palm into their nose.
5) You'll miss your target and drive t your palm into their forehead or eyes, driving their head back.
6) You'll totally miss and end up in a collar hold, plum, or grabbing their hair. Allowing you to breakdown their posture or throw them.
This is also a great way to initiate/intercept the ambush, giving you a great oppurtunity to build up momentum.