Deutsch term (edited): Einschn�rwalze

A guess; perhaps it's a lateral roller the constricts the effective width of the belt?
I think necking is more appropriate for 3D objects, such as bottles, pipes, metal bars, etc.

Cornelsen Tech WB Maschinebau / Anlagentechniek / Umwelttechniek gives the following translations:

einschnüren: (zusammenziehen) constrict; (schrumpfen) contract; (drosseln) throttle
Einschnürung: constriction; (Feurraum; Rohre) contraction; (Querschnittsverminderung) necking; (Feurraum; Rohre) reduction; (Fläche) reduction in area

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Note added at 10 hrs 36 mins (2004-05-11 00:15:04 GMT)
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New suggestion (added as a note because Proz has previously warned me that *systematically* posting multiple answers is against the rules):

snub pulley, snub roller

Background: Although Einschnürwalze gets no google hits, Einschnürrolle does -- for bucket elevators. In bucket elevators that use the \'positive discharge\' system, an extra pulley/wheel is placed slighly below the head pulley and horixontally offset, to deflect the chain or belt inward and cause the buckets to tile fully over. This has the effect of reducing the distance between the chain or belt on the two sides of the head pulley (thus eingeschnürt). In English,causing the belt or chain to wrap around the head pulley by more than 180 degrees is called snubbing, and the pulley or wheel is called a snub wheel or snub pulley.
From your context, I don\'t think that you\'re dealing with a bucket elevator, but it does appear that the roller is being used for two purposes: (a) increasing the contact area between the drive roller and the belt by increasing the amount of wrap around the drive roller (snubbing) and (b) tensionng the belt. You could thus call it a \'snub roller\' or a tensioning roller, or even some combination of the two, but the commonly used term appears to be \'snub pulley\'.

refs:

Einschnürrolle:
Becherwerke
... becherwerk), Becherwerk mit Becherseiten- wandbefestigung (Schwerkraft- becherwerk),
Langsam laufendes Becherwerk mit Einschnürrolle (Mischkraft- becherwerk). ...
www.heko.com/becher/becher_info_05.html - 7k

Bucket elevator / positive discharge:
Bucket Conveyor Belt Conveyor eBook
... The most common forms of bucket elevator are: ... positive discharge, for product requiring slower, less aggressive handling, where a snub pulley below ...
www.suttonbooks.net/hse_equipment/belt_bucket.html

snub pulley:

The Falk Corporation * Mining Industry
... For instance, the wound rotor motor caused the loaded conveyor belt to slip ... power surges defeated the belt tension created by the snub pulley, causing damaging ...
www.falkcorp.com/industry/mining-drummond.asp

Snub pulley: A pulley adjacent to a drive pulley
that increases the arc of contact on the drive
pulley to increase the effectiveness of the drive.
ww.niba.org/pdf/beltingterms.pdf

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Note added at 10 hrs 40 mins (2004-05-11 00:19:01 GMT)
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now for the typos: horixontally -> horizontally, tile fully over -> tilt fully over,