restrain vs pen up

restrain and pen up both are verbs.

Word	Noun	Adjective	Verb	Adverb
restrain	No	No	Yes	No
pen up	No	No	Yes	No

As verbs, pen up is a hyponym of restrain; that is, pen up is a word with a more specific, narrower meaning than restrain:

restrain: to close within bounds, or otherwise limit or deprive of free movement
pen up: confine in a fold, like sheep

Other hyponyms of restrain include bind, tie down, tie up, truss, fetter, shackle, enchain, pinion, shackle, impound, pound, pound, pound up, fold, ground.

restrain (verb)	pen up (verb)
prevent the action or expression of	confine in a fold, like sheep
to close within bounds, or otherwise limit or deprive of free movement
restrict (someone or something) so as to make free movement difficult
prevent (someone or something) from doing something