jmdn. kurz halten - to have a tight reign on so.

If it is equivalent to "jmdn. kurz halten", then the correct spelling is "to keep a tight REIN on so".
Is is possible that "to keep someone on the back burner / in reserve" is meant?

Correction: Is iT possible...

yes, rein - sorry
to to keep someone(sth. on the back burner - jmdn./etw. in der Hinterhand halten
Jmdn. auf Sparflamme halten - ist beispielsweise auch 'jmdn. keine Gelegenheit geben, sich zu profilieren'
Man kann jmdn. auch sexuell od. finanziell 'auf Sparflamme halten'

Merriam-Webster

throttle
Function: verb
1 ...b : to prevent or check expression or activity of : suppress

to throttle s.o. (financially, etc.) might work in some contexts.
Gabi, any more context?