ZVS (Zero Voltage Switching) means switching the 110/230VAC output when the immediate voltage is zero.
ZVS (Zero Voltage Switching) is easier to implement than ZCS (Zero Current Switching).
ZVS is a good solution for switching on/off devices such as switching power supplies. ZVS is not suitable for controlling inductive loads (transformers or motors), as it increases the probability of relay damage when such types of loads are frequently switched on/off.
110/230V outputs can be switched in these ways

Anytime
The most common method, sufficient for switching purely resistive loads, or when the switching takes place at most a few times per day. 
ZVS (Zero Voltage Switching)
Suitable for switching power supplies / computer servers. 
ZCS (Zero Current Switching)
Suitable for all types of loads and for switching more than once per day.
In the context of NETIO products

Legacy NETIO products switched their relay outputs anytime.
This turned out to be unsuitable for many applications. 
NETIO products with energy metering support ZCS (Zero Current Switching) when switching 110/230V relay outputs.
ZCS is the best relay switching technology we can provide to our customers for all kind of load. PowerPDU 4C
 PowerCable xxx

PowerDIN 4PZ

NETIO products without energy metering support ZVS (Zero Voltage Switching) when switching 110/230V relay outputs.
ZVS is sufficient relay switching technology for most load types, such as switching power supplies in IT solutions. PowerPDU 4PS
 PowerBOX 3Px