When I was there (about 10 years ago) it was all done manually, but I guess there is automation on some markets nowadays - but not on markets such as a next manager market.
If a clued up punter has a £50 bet, it will mean a candidate's price gets slashed. But a mug punter might have £500 on and...
I assume you are joking.
Either that or you are extremely naive.
I worked in the trading room of a bookmakers for a couple of years. There are punters who virtually never bet a loser on these markets.
As soon as they have a bet, the odds get slashed appropriately.
Unless the bookmaker is...