On Jiyu-waza and Zanshin (with a bit of movement thrown in for good measure)
More pre-grading ruminations.
Most Monday nights, I train at Chris's class. We usually finish with some "light" jiyu-waza, typically in line work. Most of the training is single attacker, so we can concentrate on the techniques. Last night though, I was attacked very quickly after "finishing off" the previous attacker in the line. My life didn't end and no-one was injured too badly, but it definitely caught me off guard.
Jiyu-waza at grading gives you an opportunity to let loose a little. You're not confined to a particular technique, and later, you won't even know what attack you'll receive next. So you have to develop the ability both to apply a good controlled technique and to "improvise" slightly when unexpected things happen. It also lets you pull out that special move you've been saving to impress your partner/spouse, mom or kids! :-)
But Jiyu-waza, especially with multiple attackers, is also probably the closest thing we practice in the dojo to a "real" fight. If you haven't heard the term Zanshin before, now's a good time to think about it. Focus on uke, but also be aware of your surroundings. Look out for potential threats (ok, not so likely in the dojo), obstacles (walls, Sensei, other nage and uke) and opportunities (bonus points for taking out a few spectators with a flying uke :-) )
Finally, move! Change the distance, rhythm and speed. This is important for a single attacker but absolutely crucial for more than one.
Quick note for Ikyu, Nikyu and Sankyu applicants
You can't choose the attack, but you can certainly suggest it. For your jiyu-waza, make sure it's clear to your uke which grab (katate, ryote or morote) you want to demonstrate next.