With his Robirds, Nijenhuis had to figure out which parts of flapping-wing flight he could actually simulate. The big concept ended up being flexibility. Instead of just flapping from one joint like a rigid two-by-four, bird wings deform across their entire length as they move through the air.For the Robirds, Nijenhuis complemented the basic hinging motion with a pitching motion on the wing tips–the further outward you go, the more the heavy-duty foam wings deform upwards and downwards. The result, when paired with some on-board sensors and sophisticated stabilization software, is a fairly convincing approximation of bird flight.
The video below has four parts to it: the first shows the difference between the robotic octopus swimming with just flexible arms, and swimming with just flexible arms in addition to a web. The most obvious difference is the speed: just over 100 millimeters per second with arms only, and up to 180 mm/s (or 0.5 body lengths per second) with the web. This is a significant increase, obviously, but what's more important is the overall cost of transport (CoT), which is a measure of the efficiency of the robot (specifically, the ratio of the energy put in over the resulting speed). The CoT for the arms-only version is 0.85, whereas the web drops that down to 0.62. So yeah, having that web in there is better in almost every way.