Interesting. First I've ever heard of Select and Behind.
Is Select essentially equivalent to {(⊂⍺)⌷⍵}, and Behind something like {(⍺⍺ ⍵)⍵⍵ ⍵} for monadic ⍵⍵ and {(⍺⍺ ⍺)⍵⍵ ⍵} for dyadic?