& is a monadic operator with an ambivalent derived function. & spawns a new thread in which f is applied to its argument Y (monadic case) or between its arguments X and Y (dyadic case). The shy result of this application is the number of the newly created thread.
When function f terminates, its result (if any), the thread result, is returned. If the thread number is the subject of an active ⎕TSYNC, the thread result appears as the result of ⎕TSYNC. If no ⎕TSYNC is in effect, the thread result is displayed in the session in the normal fashion.
Note that & can be used in conjunction with the each operator ¨ to launch many threads in parallel.
÷&4 ⍝ Reciprocal in background
0.25
⎕←÷&4 ⍝ Show thread number
1
0.25
FOO&88 ⍝ Spawn monadic function.
2 FOO&3 ⍝ dyadic
{NIL}&0 ⍝ niladic
⍎&'NIL' ⍝ ..
X.GOO&99 ⍝ thread in remote space.
⍎&'⎕dl 2' ⍝ Execute async expression.
'NS'⍎&'FOO' ⍝ .. remote .. .. ..
PRT&¨↓⎕nl 9 ⍝ PRT spaces in parallel.