tested on 515e platform 7.2

as indicated by eloy, this only works if the enable password is blank. not sure why the backspace works, but a simple enter key will get you to enable mode just as easily

with an enable pwd set:

c5-515E-pix> ena
Password:
Invalid password
Password:
Invalid password
Password:
Invalid password
Access denied.
c5-515E-pix>