@schlatterbeck:
Examples, hope this helps. :)
Basics:
1) first AND second keyword:
  /demo/issue?%40action=search&keyword=1,2,-3
2) NOT first keyword
  /demo/issue?%40action=search&keyword=1,-2
3) second OR third keyword:
  /demo/issue?%40action=search&keyword=2,3,-4
  should be equivalent to the old implicit OR behaviour
  /demo/issue?%40action=search&keyword=2,3

More involved:
4) testing together: ( first AND NOT third ) OR fourth keywords
  /demo/issue?%40action=search&keyword=1,3,-2,-3,4,-4