How is genotype imputation used in variant calling pipelines

In the supplementary of phase 1 1000 genomes they say:

"The merged genotype likelihoods were used as the input to run the
BEAGLE software"

In the more recent recalling of 1000G on GRCh38 they mention a similar approach

The VCF file containing the genotype likelihoods ...were processed in
parallel by Beagle 14 by using the following command:

java -jar beagle.08Jun17.d8b.jar 
  chrom=$chr:$start-$end 
  gl=$chr.biallelic.GL.vcf.gz 
  out=$chr.$start.$end.beagle 
  niterations=15

Little baffled because usually I think of genotype imputation using a reference panel. Are the haplotypes where where enough coverage existed being used to determine variation in those haplotyes where not enough coverage existing to call by observation?