Next is called a slingless transition to your sidearm.
Imagine that you have a sidearm in your holster, and you also have a rifle, but it is not slung at the moment for whatever reason (break, eating chow, etc...).
You are engaged by a threat, and you need to act fast. You grab your rifle and return fire with a quickness. You run out of ammo in your rifle, so you decide the best thing to do is continue fire with your sidearm until you can reach cover to reload your primary weapon. You have no sling, so how can you perform the transition without completely dumping your rifle?
Picture one is the initial phase of the transition. You have lowered your rifle to the ready position, and you wrap your weak hand over the top of the receiver of the rifle.
Picture two is the second phase of the transition. You swing your rifle around to the weak side, while keeping the rifle retained close to your body. At the same time you are reaching for your sidearm, also you should be "clearing leather" at this point.
Picture three is the last phase of the transition. You have the rifle secured on your weak side, and you begin engaging the target with your sidearm in an aggressive one-handed stance. This will give you the ability to return fire while moving to cover.
This stances are adaptable, and you need to determine with one works the best for you. There is no "cookie cutter" stance / position that will work for everyone 100% of the time. Experiment, and do your research.
Above all, stay safe, secure, and informed.