descending in holding
hello buzzc152,
when instructed to descent while in the hold, you do this immediately. remember that generally, when atc issues holding instructions, it is because there is too much traffic that wants to land at the same time. it's kinda rush hour as with your car. to accommodate the traffic situation most of the aircraft are stacked in holdingcircuits at 1000feet intervals. then, when the lowest aircraft gets it's clearance to start the approach, it leaves the holdingpattern, usually on radar vectors. then atc issues a descent clearance for the aircraft 1000ft above & all other aircraft are then descended in steps of 1000ft, kind of a cascade effect. it is vip you follow strictly descend clearances so as not to disrupt the system. atc will issue a clearance like this:" speedbird123 descent 6000ft, call when leaving 7000ft". so, you acknowledge the clearance & you use vertical speed, rod 1000ft/min & at 6700ftdescending you call:"sb123 leaving 7000 for 6000ft".(an altitude or fl is considered "occupied", +/-300ft in non rvsm airspace).
vertical speed is preferred to level change in order not to play havoc with the tcas systems.
also, when a lot of aircraft are stacked up in the holdings, atc radar screens are blurred with to many overlaying symbols & thats the reason they ask you to report "call when leaving".
there is a lot more to be told about holding procedures, but thanks to the fms, if programmed properly, it's relatively easy. a last thing i think about, if in the sim you get holding instructions, don't forget to ask for an expected approach time(eat), otherwise the devil in the back(the sim instructor), will be springloaded to give you a complete radio failure while in the hold.
hope this satisfies your question?