When I use my Ultimate G for the mattress
I unzip the permanent bag, pull out the disposable bag, rearrange the dirt in the bag so it's not so bulky at the bending point, then bend and attach to the portable handle. Then I put the Kirby on the mattress, put it on the lowest setting, engage Tech Drive, and go. I leave the brush roll engaged for that deep-down cleaning.
I always let Tech Drive do the work because the combination of the powerful suction and the weight do make it harder to handle as a portable. I've read Kirby manuals that say not to use Tech Drive in the portable mode. I say YEAH RIGHT. 23 pounds of weight combined with Kirby suction force, make Tech Drive a necessity on mattresses.