I'm not quite sure I understand the Contingent Summoning description either. It says that "he gain the spell like ability to use the highest memorized summoning spell once per day as a spell like ability. If no summoning spell was memorized the ability doesn't work."
Does this mean that if I memorise a Summon monster IV as my highest, I'll get a spell like ability called by the same name, but it's not subject to concentration failure if I get struck while trying to use it? What happens when I use the spell like ability, does the memorized spell vanish and vice versa?
Looks like a very interesting class!