I believe it will have something to do with which mode the A/T annunciates:
SPD: it will control speed to whatever it was when ALT CAP was annunciated, this is always an IAS number too, which is annoying sometimes when your flying Mach No and getting step climbs
EPR/N1: it will maintain the reference EPR/N1 setting, this is a risk with low level ALT CAP, as you can level off with Takeoff EPR/N1 still set and very rapidly accelerate leading to bust flap limits etc if your not aware of it..
As for low speed protection, ie if you were climbing near to stall speed or something like that and pulled back just before it went into SPD / ALT CAP, never been in that situation, so don't really know, interesting question though
757JJ