Q:

How does gas in accretion disks of supermassive black holes create orbital torques on smaller black holes within the disks causing them to migrate? 

This answer to What enhances the capture and merge rates of pairs of small black holes orbiting around supermassive black holes?  links to Migration Traps in Disks Around Supermassive Black Holes which includes the following sentence in the introduction:

Migration toward trapping orbits may be such a mechanism.  Objects orbiting within differentially rotating disks exchange angular momentum with the gas around them as they orbit, which results in a torque, typically causing the objects to migrate. 

Question: How exactly do density gradients in accretion disks of supermassive black holes create torques on smaller black holes orbiting within the disks?  Is it through simple gravitational forces, GR effects (maybe some "spin-orbit coupling" thing?),  aerodynamic drag, accretion by the small black hole in a velocity gradient, something else, or some combination of these? 
One sentence in the linked answer is

These "migration traps" (Bellovary et al.  2016) are caused by torques exerted on the orbiting objects (in this case black holes) caused by gas accretion. 

and I think it means that the orbital torque (torque on the small black hole about the SBH's center) comes from gas accretion by the small black hole.  If this is the case, is it possible to elaborate a bit? 

A:

Not really my area, but this question is probably related to the planetary migration in circumstellar disks.  In this case, the migration is caused by gravitational interactions between the planet and the gas in the disk.  There are two explanations for this effect

Impulse approximation: consider a parcel of gas in the corrotating frame.  If the gas is close enough to the orbit of the planet in an internal orbit, it will eventually reach the planet and its gravitational pull will move it to an external orbit.  From the change of velocity in the gas, you can calculate the torque exerted to the planet.  This approach is relatively simple in terms of math, but uses a lot of assumptions which are not true.  However, it can be used to get an order of magnitude estimation. 

Resonant torques: Decompose the torque exerted to the planet in the sum of individual torques at resonant locations in the disk.  Much more complicated, but better results. 

You can find the complete deduction in Astrophysics of Planet Formation by Armitage, or the compact version in this paper. 
There are surely differences between planets and black holes (GR corrections, larger accretion rate, etc.),  but I'd assume the physical principles are basically the same

