'HOLD' mode means just that, HOLD the thrust levers as your rate of descent, speed and distance to the selected level is under your right hand.
By selecting the thrust you wish (the autothrottles being 'clutch' disengaged to prevent the autothrust messing about with your pilot selected thrust) you have the ability to control your speed and descent thus achieving clearances as requested by ATC. Pretty nice system if you wish to play with it.
The autothrottle will wake up at ALT capture and re-engage, if you disconnect BOTH flight directors the auto throttle will wake up in SPD mode. If you deselect only ONE FD and then level the aircraft before ALT capture mode then you're in trouble of your own making.
It's not a 'trap' per se, it's a 'feature' that is useful as long as you are aware of the limitations and, as has been said here previously, it's not a mode you should be using close to the deck and on finals.
If in doubt, AP off, manual flying, both FD's off then back on again if they are giving you good info otherwise both off and fly the thing like an aeroplane!