You can style the description to appear when the field is focused :
var descriptedFields = [
{type: 'select', label: 'A select', description: 'Select one', choices: ['One','Two','Three'] },
{label: 'StringField', description: 'Here is the description', value:'Jacques' },
{type:'boolean', description: 'Check this box if you are happy to be there', label: 'Happy to be there ?', name: 'email'},
{ type:'url', description: 'Your blog url', label: 'Website', name:'website'},
{ type: 'radio', label: 'Radio', description: 'My favorite is YUI', name: 'example1', choices: ['Ajaxian','YUI blog','other']},
{ type: 'text', label: 'Comments', description: 'Enter your comment'}
];
new Y.inputEx.Group({id:'sampleGroup', parentEl: 'container1', fields: descriptedFields, legend: 'User Informations'});
/*
CSS :
#sampleGroup div.inputEx-description {
display:none;
}
#sampleGroup div.inputEx-focused div.inputEx-description {
display: block;
}
*/