Telecasters allow players to select the neck and bridge pickups together, but Stratocasters don’t. Many players over the years have added switches to Stratocasters to allow this combination, and the switch is usually a push/pull pot or a mini-toggle switch (manufacturers - including G&L - have offered this as stock wiring on guitars).
Lindy Fralin had a better idea. Rather then use a switch, he uses the bottom tone control to function as the switch. But since this control is a variable resistor, rather than merely switch a pickup on or off, it allows the player to “blend” a pickup into or out of the mix, like a volume control would do.
In the wiring diagram linked above, you can see that in certain switch positions the neck and bridge pickups fade from white to orange. These pickups can be on, off, or anywhere in between, by turning the blender control counter-clockwise. In the full-clockwise position the blender control does absolutely nothing, and in fact makes itself disappear from the circuit so as not to load the circuit (think true bypass). In this position, the 5-way switch functions like a normal Strat, with one volume and one tone control. Turning the control counter-clockwise in positions 1 and 2 blends in the neck pickup, while turning it in positions 4 and 5 blends in the bridge pickup. The player can get lots of subtle tonal coloration with this control. (The “clockwise=off” rationale is that this is typically the default control position, and we wanted the blender to be removed from the circuit in the default position).