There is a gradual slowing of active growth. Deciduous trees loose their leaves. Buds develop a protective coating as lower temperatures herald the winter season. Bonsai become dormant in winter the same as all the trees in your landscape. It is important to keep your bonsai dormant until late March.
Warm temperatures encourage the start of spring growth regardless of the season.
Once a tree breaks dormancy it is not possible to reverse the process. Keeping trees dormant can be difficult when temperatures fluctuate widely. Then when temperature drop the new growth is killed. Thus, it is necessary to keep the root system cold. Mulching after the soils is frozen will help.