Hydrogenation saturates the double bonds, alkene back to alkane, and raises the melting point of the products. The double bond oxidatively adds to the nickel metal catalyst. This gives you cis starting materials again. Reverse the reaction, reductive elimination, after a 180 degree rotation of the chain. This gives you trans isomers back.
Hydrogenation is run to only partial completion.
As a result there is a detectable content of isomerized double bonds in the still partially unsaturated product. The long polymer molecules are twisted into helices by their periodic cis double bonds. The result is springy rubber. Gutta percha is trans-polyisoprene. The long polymer molecules tightly stack as their planar zig-zag configuration fits togeher like a brick wall.