It should always be steady to flush. That does not mean it has to drop or sit and given that it should also be able to mark the bird when it has been shot, it needs to stop in a position to watch the bird fly away.
When working thick cover or in woods, the dog may need to move a little to keep the bird in sight, so stopping may not be “immediate”.
The spaniel must stop of its own volition and not have to be stopped by the handler’s whistle.
Some spaniels may stand on their back legs to watch a bird away when flushing in tall cover, but, so long as there is no forward movement, steadiness to flush has been achieved.