This is the BabyJogger Maxi-Cosi car seat adapter in place, we believe following the instructions. The seat latches onto the top of the adapter using a latch in the seat.
The instructions are adamant that the red lever is in the down position in order to lock it in place. Looking at the mechanism the red lever pushes a piece of plastic into the hole, but there’s nothing in the hole to lock.
Looking online there seem to be a few different models of car seat adapter, some of which have a metal frame to hold the car seat. This metal frame fits into the holes. The instruction leaflet for our adapter lists the items included, and does not include a metal frame.
It looks as if this plastic part is a single part that is used in all of the different adapters, and the note in the instructions is left over from the models that use the metal frame. The two straps would also make more sense with the frame type – to hold the seat onto the frame. Our seat has latches, so the straps are an extra safety feature for us.