How were drawbridges and portcullises used tactically?
I was recently asked why medieval castles would have both a drawbridge and a portcullis. After giving an explanation, I realized that it was just an assumption, and I had no sources for what I’d said.
For as long as I can remember, I’ve assumed that the reason for having both is that each had a complementary action – the portcullis could block the entrance very quickly (under gravity) whilst the drawbridge could open the entrance (i.e. be lowered) just as quickly. Using the two together would give the defenders a method of letting small groups of allies through the defenses with little chance of the attackers breaking through. If the defenders want to (for example) send out a raiding party, they can start by slowly raising the portcullis, then at the right moment drop the drawbridge, let the group out, and then rather than having to rely on slowly and laboriously winding the drawbridge up to secure they castle again, they can release the portcullis and be secure again in a fraction of a second. Once the portcullis is down, the castle is secure they can take their time winding the drawbridge back up. Similar idea for when the raiding party comes back.
So, is the above just a mis-remembered story on my part, or is there any truth to it? How would drawbridges and portcullises be used together, and are there any explanations in the historical record?