The modern bows the violin and viola players are using date from only the 19th century. They were invented by François Tourte. They are very light and use a "recurved" design to hold the horsehair in high tension.
In Bach's time, the bows were simpler, more like an indian "bow and arrow" bow. The player kept the hair tension high by inserting his thumb between the hair and the bow and applying pressure on the hair while playing. It was much more difficult to play this way, but it did allow the player to adjust the bow hair tension during the course of playing. You can't do that with a modern tourte bow.
In Bach's time, the string player could play the chords you see in Bach's music by relaxing the bow hair so that it played several strings at once - the hair curved over the strings and touched several at once when relaxed. The high tension tourte bows can't do this so the Bach chords are now played as rapid arpeggios. This is another way the music sounds slightly different than it was in Bachs time.