But first, go back in the film to when Bowman and Poole are working inside the CM. They are taking notes, when Poole finishes and gets into the pilot's seat. What does he do when there? He grabs a seatbelt from his right side and secures it to a buckle on his left, just like you would in a car. You hear a metal-to-metal sound when he does so. Now, flash forward to Bowman about to blow the hatch. What does he do before turning around to face the hatch? He unsecures something that we assume is a seatbelt. But it is an odd action for a seatbelt; he reaches down to his right and we hear a metal-to-metal noise. Ok...he's unbuckling himself. His next action SHOULD be to return the buckle to his left side, like helping a spring loaded belt re-spool into its container, again like a car's seatbelt, or like the one in the CM. But there is no seatbelt latch in his hand as he reaches over to his left and then oddly repeats the same action he did on his right. Again, we hear metal-to metal. So in order to get out of the seat, he had to do a two-step operation instead of one, without a belt being used in the process, as far as we can tell.
Now, I have been wondering what those two "posts" are that straddle the legs in the pod, that have three pushbuttons each on them, usually hidden from view. They line up exactly with those rectangular metal fittings on the outside of each leg. I have originally thought that like most spacesuits of the time, these would have been emergency pressure relief valves. Generally, you can clearly see these on suits of the time, but circular and lower down and in the front of the shins. You don't see these when the suit's thermal layers are covering them. 2001 suits also have similar metal fittings on the upper arms. So this led me to the possibility that the two leg fittings are what secure the astronaut in his pod seat. Otherwise, Bowman's two-step action makes no sense. Also, his overall arm motions indicate he could be pressing one of those post buttons on each side. Also, such a restraint system might make sense for the pod since it is a very low-thrust vehicle. All this is pure conjecture, especially since the moon suits have the leg fittings, too. Why would THEY need them? For the Moonbus? Apparently the bus has standard belts.
So what's going on with Bowman's actions, and the two resulting metal clicks? We will probably never know for sure. At any rate, having four pressure relief valves on a suit (two on the arms, two on the legs) is a bit of overkill.