Load Shave function

Depends on how you have it set up and there will be limits.  For example, it cannot shave beyond inverter wattage capability.  Also, limits are set on battery usage and its state of charge.  

In the limit case, you move into time of use shifting function to work around time of use tariffing by utility company, where all power for loads come from batteries, again under the limits of inverter maximum power capability and battery state of charge.

If you understand the inverter is a bi-directional AC to DC or DC to AC converter, and it sync's and runs in parallel with grid and AC output loads you can understand the options easier.  If inverter PWM raises its AC output voltage a little bit it pushes power from batteries to AC node.  If inverter PWM is adjusted to be a little lower than AC node voltage it takes power from AC node and pushes it to batteries.  

By AC node I mean the inverter AC, AC input, and AC output which are all tied together  There are current sensors on AC input and current sensor on inverter AC current. Knowing these two of three items currents, the unit can tell how much current is going out the AC output port.  See diagram below.

With this, just by adjusting the inverter PWM duty cycle, to raise or lower inverter AC voltage, it can impact which direction the AC current flows.  By bringing AC input current to zero the inverter is supplying all the AC out loads or it can only push enough to reduce AC input current by a given amount, or it can push harder than AC output current requirements which results in pushing power to grid along with supplying all the AC out loads.  Turn sell to grid off means the same thing as not allowing AC input current to go beyond zero which is back feed direction.  The voltage sense and current sense phasing measurement tells its flow direction.

When you set a maximum AC input current limit, the inverter with use up to that limit to supply AC output load and battery charging.  When AC out load is less than AC input limit then anything left over to AC in max limit is available for battery charging.  If you set charging configuration to be greater than the AC input current limit can provide then inverter automatically backs down charging.  AC output load has priority over battery charging for max limits set on AC input  (typically used when running on a generator to avoid overloading generator).  The inverter PWM adjustment can also supplement generator power so AC output load could exceed what the generator or inverter could supply individually.

There is also AC coupling where a PV grid tied inverter is placed in parallel with hybrid inverter's AC output.  Excess PV power can go to charging batteries or just pushed through to grid. (up to limit of AC1 connect relay amperage rating).

Its all feature firmware in the programming that uses inverter PWM adjustments to vary inverter AC voltage to battery voltage ratio.  The confusing part of selecting options is there are dependent interactions (like max AC input and charger level settings) that are not always well explained in manual.  Knowing the basic functionality of PWM inverter voltage control can help you understand these interactions.