Ampersand is a Monadic operator with an ambivalent operand
Operator Ampersand means
delay←{'Delayed: ',⎕DL ⍵} ⍝ delay function
delay 10 ⍝ delay for 10 seconds
Delayed: 10.2228
⎕←delay&10 ⍝ delay for 10 seconds in new thread 1
1
2+3 4 ⍝ execute something in current thread
5 6
⍝ thread 1 completes:
Delayed: 10.03183