If you try it you will see that the tension is not that high. This is a grip the string, pull it, then relax kind of technique. Use the weight of the bow rather than any muscle. This is why you need to do the slow spiccato close to the bridge. Faster spiccatos are easier because they bounce by themselves, even slightly faster. Learning this type of spiccato is also good for your finger flexibility. This exercise is more for practice than performance. The faster example is more what you will achieve in performance though probably not as fast.

When playing double stops you don't want a more slack bow. Twice as many strings require a little more pressure to vibrate. You should only have enough tension that the string will vibrate unless you need a strong dynamic.