Let's get scientific.
From Wikipedia (which I adore):
Jumping spiders are generally diurnal, active hunters. Their well developed internal hydraulic system extends their limbs by altering the pressure of body fluid (blood) within them. This enables the jumping spiders to jump without having large muscular legs like a grasshopper.
When a jumping spider is moving from place to place, and especially just before it jumps from one place to another, it tethers a filament of silk to whatever it is standing on. Should it fall for one reason or another, it climbs back up the silk tether.
Cool images worthy of a COOL spider.