They can transport themselves anywhere in the world at any time, for any reason. Due to a genetic anomaly, certain quantum mechanical interactions in the nerve cells of a Jumper create microscopic inter-dimensional apertures between this universe and another universe where physical laws as we know them do not pertain.
You should leave jumper cables connected for around five minutes at first. If your car still doesn't start after five minutes, it may take longer up to 30 minutes in extremely cold temperatures. If after 30 minutes your car still doesn't start, there's likely more wrong than simply a dead battery.