Validation in NW 1.5.X

[Good Greeting time] NWers,

<hypothetical> 

 I have a form with 3 fields

• Title (usual single line of text field, 255 chars)
• Check1 (a yes/no field, default = no)
• CheckText (a single or multiple lines of text field)

Scenario 1

• Open a new form in 'New' mode.
• Enter a value in title field, select save.
• Item saves as expected

Scenario 2

• Open a new form in 'new' mode
• Enter a title value
• Check the yes/no field, checked
• do not enter any text in the CheckText field
• Select save
• Form presents a validation error message informing the user that if Check1 is selected, text must be entered in CheckText (ie; not blank).

</hypothetical>  

Helpful clues greatly appreciated.