This spin is much harder to perform and even harder to explain than the previous three. Only attempt when you have completely mastered both the regular spin and the top spin.
Hold the rifle in right flat. Your first spin will be a normal right-handed spin. Upon bringing the rifle up for your second spin, receive the strap side of the barrel in your left hand, thumb pointed toward the tip of the rifle (the place your left hand would be on the rifle if you were doing normal left handed spins). Continue to rotate the rifle in the same counter-clockwise direction by letting go with the right hand. Receive the rifle back to the right hand on top of the barrel (the position it would be in while performing right handed top spins) and then to the left again on top of the neck (the position it would be in while performing left handed top spins). After this step, the rifle should return the neck to the right hand and the process begins again. It should have a bottom-bottom-top-top rhythm with each grab and you will always use both hands.
On the left side the process is similar, but starting out in left flat. Make one normal left handed spin, on the next spin use the right hand to grab in the normal right handed spin place on the neck, receive back to the left on the top of the butt, then with the right on the top of the barrel. Again, a bottom-bottom-top-top rhythm will be acquired.
Work toward achieving these one-handed.