Spreading from one to two hands and back

[NB10]

I would like to understand why a player would spread from one hand to two hands and when they would drop back to one hand and why. Without regard to heat, but optimal play, is it true count, penetration, number of players ? I am sure there is no exact answer so I and looking for other players opinions and ideas that would create EV, without considering the pit or eye in the sky.

I believe Blackjack Attack recommends that at a high TC (+5) you should play one hand if you are playing heads up, and two hands, each at ~73% of max bet, for all other numbers of players at the table. Someone correct me if I have misinterpreted this..