Last year I went to a Port quinta festival where grapes are trod in lagar. It's a fantastic experience - a 200-year-old ritual for pickers and cellar workers, who link arms and have a fun time. Port producers need to extract as much colour and flavour as possible from the grapes before fermentation starts, and treading is the gentlest and most effective way to do this. The big problem in the Douro is labour. Only three per cent of Port is produced in lagar, because of this.
Never "between the toes", but with the flat of the feet. However, the practice is now largely confined to festive rituals. Even the Portuguese have to make an effort to be economically competitive these days!