constrain vs pen up

constrain and pen up both are verbs.

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

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

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

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

constrain (verb)	pen up (verb)
severely restrict in scope or extent	confine in a fold, like sheep
to close within bounds, or otherwise limit or deprive of free movement
compel to behave in a certain way