Yes, this is where the X% of Y% stuff comes into play. Open up the Task Assignments inspector for the tasks you want to have worked on at less than full effort, and adjust the first number down, as appropriate. In my example below, I use 50% of 100% which means that 1h of work will occupy 2h on the timeline, and that 100% of the hours in the resource's daily schedule will be available.
A few things to note:
the percentages shouldn't be multiplied together, as they don't work that way. The second number is the percentage of the hours in the day that can be allocated, and the first number is also a percentage of the hours in the day. 100% of 100% means all the assignable hours for that day, working exclusively on one thing. 50% of 100% as the setting for two independent tasks will cause the entire day to be spent working on those two tasks, effectively 30 minutes per hour on each one. A task that takes 4 hours of effort will have a duration of 8 hours on the timeline. 75% of 75% means only 6 hours out of the standard 8 hour day are being assigned, and they are all being used for that one task. To split that 75% equally over 3 tasks, they would each get 25% of 75% assigned.
If you have a fractional effort assigned, but there's only one task scheduled, the timeline will show that resource "loafing" along at only partial utilization, and it won't assign part of a full-intensity task to occupy the available effort. You can see this in my example for Task 2 after Task 1 completes.
Hopefully, this hasn't confused the situation more than it already was :-)
