Quote from Swish:
TRIN is calculated from the cumulative trade info through the day. Thus there is no moving average or reduced time frame for calculation. That's why TRIN can be misdirecting in certain situations - for instance:
1. On big gap up or down days, intraday moves will be masked since TRIN is based on previous close. If MSFT gaps up $2, then declines by $1.90 intraday, it's still shown on the advancers side of the calcs.
2. The market's been up all day, but starts tanking in the last hour. It will take longer for TRIN to be significantly impacted if compared to had the action occurred early in the day.
Thus, after the first part of the trading day, most folks put more emphasis on the direction rather than the value of TRIN.