Very often players have to change very quickly from bowing to plucking and back again. It is easy to play a bowed note and then immediately a plucked note if the bowed note finished near the heel of the bow (the end where the bow is held). If the bowed note finishes near the tip the player needs a moment to get the hand ready to pluck. It can take a little more time to go back to bowing again because the player has to get the bow back into playing position.