Various bits of what is known about busybox shells, in no particular order.
2008-02-14
-ash: does not restore tty pgrp if killed by HUP. Symptom: Midnigth Commander
+ash: does not restore tty pgrp if killed by HUP. Symptom: Midnight Commander
is backgrounded if you started ash under it, and then killed it with HUP.
2007-11-23