When using High Speed Divine Words, it appears to be refunding all momentum used on the cast.
As an example, if you cast HSDW, and then activate an invoke on the same turn, it will spend your momentum as normal. However, upon next turn, casting the invoke is then completely free as it seems to be refunding the momentum spent.
This only seems to work with invocations from Evoker, but I only tested this with Arctic Claw (Evoker) and Lux Relido (Solblader).
As an example, if you cast HSDW, and then activate an invoke on the same turn, it will spend your momentum as normal. However, upon next turn, casting the invoke is then completely free as it seems to be refunding the momentum spent.
This only seems to work with invocations from Evoker, but I only tested this with Arctic Claw (Evoker) and Lux Relido (Solblader).