We want to avoid someone beeing able to vote twice with the same tokens:
- Alice has 10 tokens
- Alice votes
- Alice send the 10 tokens to Bob,
- Bob votes
→ the 10 token have produced 20 votes, this is bad.
The solution for that is to use snapshots. You vote with the tokens you had at the moment of the snapshot.... more precisely, you vote with the tokens delegated to you at the moment of the snapshot.
In the governor, the snapshot is when the vote start. It can be at the moment when the proposal is made, or it can be latter (see the vote delay in the documentation).
In other to vote, the delegation must happen BEFORE the snapshot.