The picture is just half of the information. Give us the link to where you found it.
We (you) need to read the context.
Currently we don't know whether it shows
* current
* voltage
* duty cycle
* anything else
The slope is the di/dt = V/L where L is the coil inductance. V is the DC bus of the 3Phase brij.
Remember in a BLDC , you have the coil to coil commutation...and also, the high frequency current regulation commutation within that. Basically a 3 phase bridge is 3 step down buck converters, which control their output current.