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This is how I adjust people's watch band lengths.
The watch in the video uses cheap cotter pins, which aren't solid, but have a split on one of the ends. When adjusting, you don't want to push the split end all the way through the link's length, so push from the 'solid' side, outward, which is what the arrows etched in the bracelet are supposed to indicate - the direction to push the pin outward. The split piece should be the first to go out.
To put links back together, reverse the operation, remembering to not push the split (weaker) end through the entire length of the link connection, but instead, the split piece should be the last to go in, and this reverse direction goes against the direction of the arrows on the links (solid end first, at the head/tip of the arrow on the link, and then push from the split side backward from where the arrow points).
This method works for friction pins too, such as on the old Omega Seamasters (2255, 2531, 2254, etc. and newer until they went to screws). Usually on friction pins there's a notch in the pin that's to one side of the pin - not in the middle, but closer to one side. Similarly as described above, you don't want to push that notch all the way through the connection, so make it the first out (as per the arrows), and the last side in.