6.86 Use of ## &[]

reference

To understand what it does, these three do the same thing

Map[If[# == 1, Unevaluated@Sequence[], #] &, {1, 2, 3}]; 
If[# == 1, Unevaluated@Sequence[], #] & /@ {1, 2, 3}; 
If[# == 1, Sequence @@ {}, #] & /@ {1, 2, 3}; 
If[# == 1, ## &[], #] & /@ {1, 2, 3};
 

All above give {2,3}. So the effect of ## &[] is to remove the entry completely (so we do not end up with a Null or empty slot in there).