MacOS – Associate a file extension with an app in Snow Leopard

You can use the H keyboard shortcut while the presentation is running. This (according to the help displayed using the ? shortcut) is used to "Hide the presentation and switch to last application used".

If you make sure the application you are wanting to demo is the last application used, you can do the following

1. Go through your presentation until you get to the Demo slide
2. Press H to switch to your application
3. Demo the application
4. Use ⌘ + Tab to switch back to Keynote
5. Your presentation resumes from the Demo slide.

NOTE: See (also) a corrected/improved answer in the question itself.

Following solution worked for me:

Open a terminal and backup com.apple.dynamic_pager.plist which you're going to change in a second:

$ cd /System/Library/LaunchDaemons
$ sudo cp com.apple.dynamic_pager.plist{,_bak}

convert binary plist to xml:

$ sudo plutil -convert xml1 com.apple.dynamic_pager.plist

and open it with your favorite text editor

$ sudo vim com.apple.dynamic_pager.plist

it'll look something like this:

 1 <?xml version="1.0" encoding="UTF-8"?>
 2 <!DOCTYPE plist PUBLIC "-//Apple//DTD PLIST 1.0//EN" "http://www.apple.com/DTDs/PropertyList-1.0.dtd">
 3 <plist version="1.0">
 4 <dict>
 5     <key>EnableTransactions</key>
 6     <true/>
 7     <key>HopefullyExitsLast</key>
 8     <true/>
 9     <key>Label</key>
10     <string>com.apple.dynamic_pager</string>
11     <key>OnDemand</key>
12     <false/>
13     <key>ProgramArguments</key>
14     <array>
15         <string>/sbin/dynamic_pager</string>
16         <string>-F</string>
17         <string>/private/var/vm/swapfile</string>
18     </array>
19 </dict>
20 </plist>

In line 17 modify /private/var/vm/swapfile (e.g. /Volumes/partition2/swapfile), save and close your editor (":x" will do both in vim).

convert the plist file back to binary:

$ sudo plutil -convert binary1 com.apple.dynamic_pager.plist

After rebooting your Mac you should find the swapfiles in the directory you specified.

If you run into any problems you can restore the backup you created in the first step with:

$ cd /System/Library/LaunchDaemons
$ sudo cp com.apple.dynamic_pager.plist{_bak,}