Alternate actions are a very powerful feature of Quicksilver. When a user selects an action in the second pane, they can hit ↩ to run it, or they can hit ⌘↩ to run an alternate (if one is defined). Generally an alternate action will be similar to the main action, but modified in some way.
As of Quicksilver ß71 you can view the alternate to a given action (if it exists) buy holding the ⌘ key when Quicksilver's interface is open. You can view a full list of the alternate actions in the 'Alternate Action (⌘↩)' column of the Actions preference pane.
 Table of Actions and their Alternate Actions
|Action||Alternate Action||Plugin Required|
|Open (file)||Reveal (file) in Finder||Core Plugin|
|Hide (application)||Hide Others||Core Plugin|
|Quit (application)||Quit Others||Core Plugin|
|Toggle Application||Hide Others||Core Plugin|
|Open URL||Open URL in Background||Core Plugin|
|Search For||Search For & Return in Quicksilver||Web Searche Module|
|Spotlight Contents...||Spotlight Contents in Finder||Spotlight Plugin|
|Open (item) at Login||Do Not Open (item) at Login||Core Plugin|
|Go & Fill...||Reveal in 1Password||1Password Plugin|