You know how in the Toa Mahri minimovie, Jaller was seen with the Hahnah crab mounted onto his back? Well I've managed to find a way to accomplish this with the official set without use of any extraneous pieces.
Disassemble the chain that links the crab to Jaller. Those two axles are important to this.
Lift down Jallers chestplate and slide the two axles in facing as they are shown in the photo.
Because they are inside pinholes, the axles will, of course, dis align so hold them down with your fingers as you...
Attach the Hahnah crab to them, via these axle holes.
Following the aforementioned steps should result in this. I like this method as the Hahnah Crab sticks out less than another method that I found in a video dealing with a similar subject.
Step 4 (optional
When Jallers like this, I like to attach the chain via the two little knobs on those axles and drape it inside his chestplate. The three extra Cordak rockets can be stored on his arms and in the center of his Cordak Blaster.
That's all for now!