While she might have been mostly powerless to resist the Force, Tanya was not powerless to at the very least lessen the impact of the blow with the use of her superior cultivation base.
*BAM!!*
Elsa's impactful blow landed squarely onto Tanya's palm which had been raised to intercept the blow.
With Elsa not being the first she had actually sparred against who had gained Ordinance Harnessing, Tanya knew the Ordinance was more than capable of covering up for the one-stage lower cultivation of its wielder.
As such, she knew what to expect and the resultant backblast that knocked her against what could only be described as the boundary of the space available by virtue of the Void disk did not really catch her by surprise.
What did surprise her or at least caught her unprepared was Elsa's viciousness as she was already upon her opponent in a second and ready to deliver a decisive victorious blow.