By default, your Employees can only offer up their whole shift into the shift pool. However, if you enable "Partial Shifts" under "Company Settings > Shift Pool", your Employees will have the option to split their shift up and only offer part of their shift for other Employees to take.
Here's how to enable it:
1. Hover over your profile picture > Company Settings.
2. Now click the Shift Pool tab > check Partial Shifts > click Save.
Once enabled, your Employees will be able to offer up their shift and uncheck Full Shift. They can then modify the time to specify which part of their shift they wish to offer up.