⌷ — Index
I⌷Y selects axes using the same indices as brackets. Omitted trailing axes remain whole. ⌷Y is identity.
2⌷3 4⍴⍳12 ⍝ 5 6 7 8
2⌷[2]3 4⍴⍳12 ⍝ 2 6 10
(⊂3 1)⌷10 20 30 ⍝ 30 10
Enclose a vector to use it as one axis’s index list. [K] selects which axes the indices address.