A time system is spilling if it could happen that had a player used less time for his first N moves he nevertheless would end up in absolutely the same time-system state. What this means is that surplus time is not carried over to the next time-system state. It is lost (spilt).
Follwing time systems are spilling:
If a player in Canadian overtime has played his lot of stones for the current period, the next period with the next lot starts -- no matter how much time was left over.
Since this seems odd, two modifications exist: