(define (format-i x)
  (format #f "~a\n" x))

(define (reproduce)
  (let loop ((i 0))
    (format #t "hi: ~s\n" (format-i i))
    (usleep 1000000)
    (loop (1+ i))))
(define tth (call-with-new-thread reproduce))

if you run this code, you won't see much output, only next eval action will actually show all accumulated output.