Please can someone tell me how I can find out what the custom was for captains and customers to bargain by putting their foot on a rail. I saw this in Series 6 and would like to know what it is. I cannot find information.

I notice Onedin bargaining in the big hall where the business was conducted to acquire cargo by putting his foot on the rail together with the customer. The customer removed it when he did not like the price but put it back when they came to an agreement. I would like to know about this. Can anyone advise.