From what I gather the more secured keys should be discarded 10 seconds after lock screen event. Lower security keys stay in memory to allow background activity.
Encryption on ios, if i understand correctly, is on a per file basis. There is thus no "mount" event to look for and it should provide no value to use a less secured key if you do not intend to run on background because decryption is supposed to happen on the fly.
PS: Also if I remember correctly pressing down the emergency sequence (holding power + volume up) discard ALL keys instantly and unlock require the passphrase as if you just rebooted. Emergency call don't need to be issued just initiated (must hold 10 sec or confirm on screen to make the actual emergency call).
