To open vim at the line containing the tag foobar
type this at the shell commandline:
vim -t foobar
To jump to the same tag on the vim commandline type:
:tag foobar
If you want to split the window and jump to the tag in the new window type this in vim s commandline:
:stag foobar
If you want a keystroke that specifies "the word under the cursor", then according to this question, you can use the CTRL+R CTRL+W to get that:
:tag CTRL+R CTRL+W
You can also use :
:nmap <leader>w :tag <c-r>=expand("<cword>")<c-r>
Now typing <leader>+w
(which is in my setup, so I d press w) will be the same as typing :tag <word under the cursor>