Subject: grand tack model, outward why?

Q)  Why (=by what mechanism) can Jupiter and Saturn move "outward" in the Grand
tack model (https://en.wikipedia.org/wiki/Grand_tack_hypothesis)?
Planets moving "inward" can be explained by planets losing linear or orbital
momentum via friction with their surrounding gas but how could the "outward
migration" of planets happen? 


One explanation could be the "Jupiter and Saturn"(=1) being pulled outward by
the planetesimals (=2) orbiting outside of Jupiter and Saturn orbits via
gravitational interaction between 1) and 2)?. But how does "orbital resonance"
(of Jupiter and Saturn) play a role in reversing the migration from "inward" to
"outward"? because when Jupiter and Saturn are close to each other their total
gravitation pull gets temporarily and periodically stronger (with 2?)? (but how
do we know this is enough to reverse migration?) But if all these are the case,
planetesimals outside of Jupiter and Saturn should get closer to the sun in
exchange of the Jupiter and Saturn's outward migration? Is this the case? But I
am not sure if gravitational pull between 1) and 2) are strong enough to reverse
the inward migration of 1) to the outward migration. (because 1 seem quite
massive compared to 2(?)) How can we know if there was enough gravitational pull
between 1 and 2? How can we know if there were enough planetesimals (mass?)
outside of Jupiter and Saturn in the past? How can we know what happened to 2)?
If some of 2) fell into the sun they are no longer observable? Then, how do we
know 2 existed in enough quantity in mass?

or, when "orbital resonance" (of 1) occurs, 1 receives (orbital?) linear
momentum from 2 (because of the stronger gravitational force between 1 and 2?)
and start moving away from the sun? (and at the same time 2 loses the (orbital?)
linear momentum and start moving toward the sun?)  Is this the case? What gives?

Re: grand tack model, outward why?