[TxMt] Keystrokes to extend selection?

[Will]

Speaking of shortcuts…

Is there a keystroke to extend the bounds of a selection? I realize  
you can use the shift+movement keys for  adjustments but if you go  
forward and then back, it restarts your selection from the initial  
cursor point. I'm wondering if there's a way to lock one side of the  
selection and extend the other. Perhaps something similar to hitting  
'o' in vim's visual mode?

Sorry if I'm not making any sense!

Will